Back in 2003, when Rick Santorum was the U.S. Senate’s third ranking Republican, an Associated Press reporter asked his opinion on laws prohibiting homosexual conduct. (At the time, the U.S. Supreme Court was preparing to rule in "Lawrence v. Texas," ultimately striking down such laws in a 6-3 majority.) The senator responded:

I have a problem with homosexual acts ... [I]f the Supreme Court says that you have the right to consensual sex within your home, then you have the right to bigamy, you have the right to polygamy, you have the right to incest, you have the right to adultery. You have the right to anything. Does that undermine the fabric of our society? I would argue yes, it does. Every society in the history of man has upheld the institution of marriage as a bond between a man and a woman ... In every society, the definition of marriage has not ever to my knowledge included homosexuality. That’s not to pick on homosexuality. It’s not, you know, man on child, man on dog or whatever the case may be.

Reaction to Santorum’s now-infamous “man on dog” remarks was swift and sharp. Sex-advice columnist Dan Savage even launched a successful Internet campaign to associate Santorum’s name with a nasty byproduct of anal intercourse. Santorum, for his part, has never apologized for the remarks, although in recent years he has denied that he was comparing homosexuality with bestiality and the sexual abuse of children. During his 2012 Republican presidential campaign, he told CNN’s John King: “I said it’s not those things. I didn’t connect them. I specifically excluded them.” This denial sounds unconvincing. When Santorum said that it’s not man on child, man on dog, and so on, the not was there to distinguish traditional heterosexual marriage from a list of bad things. In his view, homosexuality clearly belongs on the bad list.

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That doesn’t mean, of course, that Santorum sees these things as equally bad. Analogies compare things that are similar in some respects, which is not the same as saying that they’re identical in all respects. If you’re in a particularly charitable mood and willing to overlook some parts of the interview, you can read Santorum’s remarks as making a claim about the logic of privacy rights: If people have the right to do whatever they want in the privacy of their own homes, then they have the right to bigamy, polygamy, incest, adultery, bestiality and so on. Or, at least, they have the prima facie or presumptive right, which could be overridden only by some stronger countervailing right, such as other people’s right not to be harmed. (That countervailing right would quickly rule out “man on child” sex, not to mention many instances of other things on the list.)

Having said that, it’s difficult to maintain a charitable mood when someone mentions your consensual adult relationships in the same breath as “man on child” and “man on dog” sex. Whatever anyone says about Santorum’s remarks as a matter of logic, they were thoughtless and nasty as a political sound bite. Even some of his fellow Republicans thought he had gone too far.

It’s not just Rick Santorum who invokes the slippery slope. U.S. Supreme Court Justice Antonin Scalia made a similar argument in his scathing dissent in "Lawrence v. Texas," as did Justice Byron White in "Bowers v. Hardwick," which "Lawrence" reversed. So have William Bennett, Hadley Arkes, Charles Krauthammer and a host of other prominent conservative writers. In the colorful words of John Finnis, those who defend gay sex “have no principled moral case to offer against ... the getting of orgasmic sexual pleasure in whatever friendly touch or welcoming orifice (human or otherwise) one may opportunely find it.”

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In the past I’ve referred to this slippery-slope argument as the “PIB” argument, short for “polygamy, incest and bestiality,” although other items sometimes make the list as well. What got me interested in PIB, aside from my wanting to defend gay people against nasty smears, is that it isn’t entirely clear what the argument is saying. Is it predicting that once homosexuality becomes more accepted (some of) these other things will become more accepted as well? Is it making a logical point, suggesting that, even if these things won’t ensue, in fact, they’re somehow related in principle? Or is it primarily a rhetorical move, simply trying to scare people away from homosexuality by invoking a parade of horribles? In many ways, the PIB argument seems more like a question or a challenge than an argument proper: “OK, Mr. or Ms. Sexual Liberal, explain to me why all these other things are wrong.” Most people aren’t prepared to do that on short notice, which makes the PIB point a debater’s dream: It’s a handy sound-bite argument that doesn’t lend itself to a handy sound-bite response.

One way to approach the PIB argument is to turn the challenge around and ask: What does one thing have to do with the other? Polygamy can be heterosexual or homosexual, and the societies that practice it tend to be the least accepting of same-sex relationships. Incest can be heterosexual or homosexual. Bestiality, I suppose, can be heterosexual or homosexual, although like most folks I prefer not to think about it too carefully. Since there is no inherent reason to classify PIB with homosexuality rather than heterosexuality, we must ask: What’s the connection?

There are two main answers to this question, and they give us the two broad versions of the PIB argument: a logical version and a causal version. (Note: calling one version the logical version does not mean that it is particularly reasonable or that the other version is illogical; it just means that the argument is based on logical connections rather than empirical ones.) Let’s take each in turn.

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PIB Argument: The Logical Version

The logical version of the PIB argument, which is the one that philosophers usually favor, says that the argument for same-sex relationships makes an equally good case for PIB relationships. In effect, it claims that the pro-gay argument “proves too much”: if you accept it, you commit yourself to other, less palatable conclusions.

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So the logical PIB argument is what philosophers call a reduction ad absurdum (“reduction to absurdity”), a way of showing that certain premises -- in this case, those establishing that same-sex relationships are morally permissible -- have absurd implications. It doesn’t matter whether approval of homosexuality actually leads to approval of these other things. The point is not to make a prediction: It’s to indicate the alleged logical inconsistency of supporting homosexuality while opposing PIB.

But why would anyone think that supporting same-sex relationships logically entails supporting PIB? The answer, I think, is that some people misread the pro-gay position as resting on some version of the following premise: People have a right to whatever kind of sexual activity they find fulfilling. If that were true, then it would indeed follow that people have a right to polygamy, incest, “man on child, man on dog or whatever the case may be.” But no serious person actually believes this premise, at least not in unqualified form. That is, no serious person thinks that the right to sexual expression is absolute. The premise, thus construed, is a straw man.

A more reasonable premise suggests that sexual expression is an important feature of human life which must be morally balanced against other features of human life. For most people, sex is a key source of intimacy. It is a conduit of joy and sorrow, pride and shame, power and vulnerability, connection and isolation. Its absence -- and especially its enforced denial -- can be painful. On the other hand, there are good moral reasons for prohibiting some sexual relationships, either individually (say, because Jack’s relationship with Jane breaks his vow to Jill) or as a class (say, because the relationship is unfaithful or emotionally unhealthy or physically harmful or morally defective in some other way). So for any sexual relationship -- and for that matter, any human action -- we must ask: Are there good reasons for it? Are there good reasons against it? There is no reason to think that the answers to those questions will be the same for homosexuality as they are for polygamy, incest or bestiality -- which are as different from one another as each is from homosexuality. Each must be evaluated on its own evidence.

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[After] our examination of the moral evidence surrounding homosexuality, [we feel] the basic case in favor of it is straightforward: For some people, same-sex relationships are an important source of genuine human goods, including emotional and physical intimacy, mutual pleasure and so on. That positive case must be balanced against any negatives -- although, as we have seen, the standard objections fall apart under scrutiny.

What about PIB? I don’t doubt that some PIB relationships can realize genuine human goods. Polygamy is the most plausible candidate: It is quite common historically, and there may well have been circumstances (for example, a shortage of men due to war or other dangers) that made it work well in particular societies. But that’s only half the story. The other half requires asking whether, despite these goods, there are overriding reasons for discouraging or condemning polygamy today. Polygamous societies are almost always polygynous, where one husband has multiple wives. (Polyandry — one wife with multiple husbands — is, by contrast, quite rare.) The usual result is a sexist and classist society where high-status males acquire multiple wives while low-status males become virtually unmarriageable. Thus, from a social-policy point of view, there are reasons to be wary of polygamy. Perhaps those reasons could be overcome by further argument, but the central point remains: Arguments about the morally appropriate number of sexual partners are logically distinct from arguments about the morally appropriate gender of sexual partners.

The same is true for incest arguments: Whether people should have sex with close relatives (of either sex) is a distinct question from whether they should have sex with non-relatives of the same sex. Some might wonder whether the problem with incest is that it poses genetic risks for offspring, an objection that wouldn’t apply to gay incest. But the reason for the incest taboo is not merely that offspring might have birth defects (a problem which can be anticipated via genetic testing and which doesn’t apply past childbearing age). It is also that sex has a powerful effect on the dynamics of family life.

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As Jonathan Rauch vividly puts it:

Imagine being a 14-year-old girl and suspecting that your 16-year-old brother or 34-year-old father had ideas about courting you in a few years. Imagine being the 16-year-old boy and developing what you think is a crush on your younger sister and being able to fantasize and talk about marrying her someday. Imagine being the parent and telling your son he can marry his sister someday but that right now he needs to keep his hands off her ... I cannot fathom all of the effects which the prospect of child-parent or sibling-sibling marriage might have on the dynamics of family life, but I can’t imagine the effects would be good, and I can’t imagine why anyone would want to try the experiment and see.

These problems apply just as much to homosexual incest as to heterosexual incest.

There is another important disanalogy between the incest ban and the homosexuality ban. The incest ban means that every person is forbidden to have sex with some people -- a relatively small group -- whom he might find romantically appealing: his close relatives. By contrast, the homosexuality ban means that gay people are forbidden to have sex with anyone whom they might find romantically appealing. Unlike the incest ban, it reduces their pool of available romantic partners to zero -- an infinitely greater restriction. One could make a similar point about the polygamy ban: In principle, any man who can fall in love with two women can fall in love with one, and any woman who can fall in love with an already married man can fall in love with an unmarried one.

What about bestiality -- Santorum’s “man on dog” example? It’s hard to know what to say here except that I share most people’s revulsion to it. Of course there’s the issue of consent. On the other hand, we do plenty to animals without their consent, including many uncontroversial interactions. While bestiality is often harmful to animals, it need not be: There’s an urban legend that comes to mind involving a woman, a dog, peanut butter and a surprise party. (Feel free to Google it.) Ultimately, the problem with bestiality seems to be less about the effect on the animal than the effect on the person, damaging his or her capacity for appropriate human relationships.

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I made this latter point in an article entitled “Homosexuality and the PIB Argument,” which prompted a reply from Christopher Wolfe. Wolfe agrees with me that bestiality is likely to damage a person’s capacity for human relationships. But he worries that I appear to “back off ” the argument and speculates that I must be afraid to say that any consensual act not harmful to others is immoral. As you are probably aware by now, I have no such fear. To the extent that I back off the personal-damage argument, it’s because I haven’t done the relevant research and don’t really care to. For all I know, zoophiles are some of the most psychologically healthy people in the world, have great sex with their (human) spouses and so on. And if that were so, Wolfe and I would have to find some other argument in order to maintain our objection or else conclude that bestiality’s wrongness is a fundamental moral fact.

Wolfe then contends that if I say that bestiality is intrinsically immoral, I open up a space for him to argue, analogously, that homosexuality is intrinsically immoral. This is wishful thinking at best, utter confusion at worst. The whole point of claiming that some action-type is intrinsically immoral is to say that its immorality does not depend on the wrongness of other action-types; its wrongness does not derive from some more general principle. It is entirely possible -- and I would add, quite common -- for someone consistently to believe that sex with animals (of any sex) is intrinsically immoral but that sex with persons of the same sex is not. Gay-rights advocates are as entitled to basic premises as anyone else. But basic premises about bestiality do not entail basic premises about homosexuality -- or any about other behavior.

I’m reminded here of a funny story from Dan Savage. Savage was on a radio show with a man who sincerely claimed to have a romantic (including sexual) relationship with a horse. At the end of the interview, as the wrap-up music was playing, Savage offhandedly said, “Oh, I forgot to ask -- is it a male horse or a female horse?” The man turned red, glared at Savage and retorted indignantly, “I AM NOT GAY!!!” (I suppose people find comfort where they can.)

One might try arguing that since PIB and homosexuality have traditionally been grouped together as wrong, the burden of proof is on anyone who wants to take an item off the list. But this response fails twice over. First, the “tradition” that groups these things together is actually a relatively modern artifact: Polygamy, as noted, was a very common form of marriage historically, and it is accepted (indeed, encouraged) in various cultures today. Homosexuality has been condemned in many cultures, but certainly not all. Indeed, same-sex eroticism has been celebrated in the art and literature of great civilizations.

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The second problem with this response is that just because practices have been grouped together as wrong, it doesn’t follow that they should be. This is especially obvious when we consider other things that were once common taboos, such as interracial sex. And while strict assignments of burden of proof may work in courtrooms, in everyday life the burden of proof (or at least, the burden of persuasion) is on whoever wants to prove something. Both sides are in the same boat there.

One might argue that homosexuality, like bestiality, is always nonprocreative. That’s true, but it doesn’t explain why polygamy (which is abundantly procreative) and incest (which can be procreative, though often with disastrous results) land on the same list. Moreover, there is no good reason to think that all sex must be open to procreation. The more one examines the PIB argument, the more it appears that opponents just lump together things they don’t like and then dare others to challenge the list. Of course, until one knows why the list members were initially grouped together, it’s impossible to offer a reason why some item doesn’t belong or to argue that the removal of one doesn’t require the removal of others.

What if the PIB-plus-homosexuality list is simply a collection of morally wrong sexual practices, each one there for its own reasons? In that case, the logical form of the PIB argument would collapse: Its whole point is that PIB and homosexuality are logically related. If the various items are immoral, but for unrelated reasons, the following analogy would apply: It would be like putting various useful objects on my desk for different reasons -- a pen for writing, a lamp for reading, a letter-opener, a stapler, a paperweight and so on -- and then arguing that if I remove the lamp I have to remove the stapler and the paperweight. Besides, [we would make the case that] the arguments for judging homosexual conduct to be immoral simply don’t work. There was no good reason for putting homosexuality on the list in the first place.

I conclude that the best response to the logical version of the PIB argument remains the simple one we started with: What does one thing have to do with the other?

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PIB Argument: The Causal Version

The foregoing discussion assumes that the PIB argument alleges some logical connection between PIB and homosexuality. There’s another possibility, however. Perhaps the connection is not logical but empirical. That is, perhaps the endorsement of one item will lead to the endorsement of others, whether or not it logically should. For instance, maybe the wider acceptance of homosexuality will embolden polygamists and make it harder for others to resist their advocacy.

This is the causal version of the PIB argument. It typically ignores incest and bestiality -- and from here on, so shall we. Because it focuses instead on polygamy, it usually refers to same-sex marriage rather than homosexual acts. (There is no such thing as a “polygamous act,” strictly speaking, although one might refer to a pattern of behavior as polygamous conduct.) So our discussion will now focus more on public policy, and specifically marriage, than on the morality of relationships per se.

The best-known proponent of the causal PIB argument is Stanley Kurtz, who claims that the slippery slope to polygamy is “[a]mong the likeliest effects of gay marriage.” Kurtz has been predicting this pro-polygamy effect since the mid-1990s. But his evidence for it is thin, and his evidence for polygamy’s connection with homosexuality is even thinner. He writes:

It’s getting tougher to laugh off the “slippery slope” argument -- the claim that gay marriage will lead to polygamy, polyamory and ultimately to the replacement of marriage itself by an infinitely flexible partnership system. We’ve now got a movement for legalized polyamory and the abolition of marriage in Sweden. The Netherlands has given legal, political and public approval to a cohabitation contract for a polyamorous bisexual triad. Two out of four reports on polygamy commissioned by the Canadian government recommended decriminalization and regulation of the practice. And now comes "Big Love," HBO’s domestic drama about an American polygamous family.

This paragraph nicely encapsulates the kinds of exaggerations and outright falsehoods that typify discussion of this issue. First, in 2006, when Kurtz wrote the above paragraph, Sweden didn’t have “gay marriage”; it had “registered partnerships,” the kind of “separate-but-equal” status most same-sex-marriage advocates typically oppose -- as should anyone worried about an “infinitely flexible partnership system.” Second, Kurtz’s case of the “polyamorous bisexual triad” was not a marriage at all, but a private cohabitation contract signed by a Dutch notary public. The relationship was neither registered with nor sanctioned by the state; it was no more a legal polygamous marriage than a three-person lease agreement is a legal polygamous marriage. Third, the fact that some Canadian studies of polygamy recommended decriminalization and regulation is hardly evidence of widespread support for the practice. And fourth, the success of the HBO series "Big Love" signaled a wave of support for polygamy about as much as the success of "The Sopranos" signaled a wave of support for the Mafia.

Kurtz’s deeper problem is that he fails to show any causal connection between these alleged phenomena and gay rights. He has tried to establish one by looking at marriage trends in Scandinavia, but his analysis falters on the fact that these trends substantially predated same-sex marriage there. (William Eskridge and Darren Spedale’s "Gay Marriage for Better or for Worse?: What We’ve Learned from the Evidence" provides a book-length refutation.)

Kurtz has also tried to establish the connection by arguing that some of the same people who endorse polygamy also endorse same-sex marriage and that they invoke the same “civil rights” language in both cases. This is true but entirely inconclusive. Some of the same people who oppose abortion also oppose capital punishment and invoke the same “sanctity of life” language, but that’s no reason to conclude that one movement leads to the other. In fact, the vast majority of the world’s polygamy supporters are religious fundamentalists who strenuously oppose homosexuality, and the practice tends to appear in U.S. states (like Utah, Nevada and Texas) with the lowest support for gay rights. Indeed, to the extent that the gay-rights movement promotes an egalitarian view of the sexes, it will likely undermine common forms of polygamy. Kurtz can link the two movements only by selective myopia.

Kurtz has also tried to connect the two issues via the issue of infidelity. He notes that polygamous societies tend to have high rates of infidelity because they promote the idea that men “need” multiple women. Such infidelity is problematic because it causes instability for wives and for children, many of whom are born outside of marriage. How does Kurtz then connect this problem with homosexuality? His logic seems to go like this: Polygamous societies have high rates of infidelity; gay males have high rates of infidelity; therefore, the gay-rights movement will lead to polygamy -- presumably by weakening the norms of fidelity that hitherto kept polygamy at bay. If we let gays marry, Kurtz seems to be saying, then straight men will start cheating on their wives. Even if this prediction were plausible -- which it isn’t -- the conclusion hardly seems justified.

First, on the prediction’s implausibility: Kurtz is basing his argument on the premise that gay male couples tend to be less sexually exclusive than either heterosexual couples or lesbian couples. Even Kurtz admits, “Lesbians, for their part, do value monogamy.” His worry seems to be that if we allow same-sex couples to marry, gay males’ sexual infidelity will bleed into the general population. But he never explains how. Keep in mind that gay men and lesbians make up a relatively small minority of the general population. Gay men make up about half of that minority, coupled gay men an even smaller subset, and coupled gay males in open relationships a smaller subset still. In Jonathan Rauch’s words, “We might as well regard nudists as the trendsetters for fashion.”

While sexual exclusivity may be challenging, it’s not so challenging that a sexually exclusive couple (straight or gay) can’t look at a sexually open couple (straight or gay) and conclude, “Nope, that’s not right for us.” After all, people read the Bible without deciding to acquire concubines. More realistically, they often encounter neighbors with different cultural mores while still preferring -- and sometimes having good reason to prefer -- their own.

Then there’s the fairness issue: The question of whether same-sex couples should be allowed to marry should no more hinge on the behavior of a subset of gay men than the question of whether Hollywood actors should be allowed to marry should hinge on the behavior of a subset of Hollywood actors. In terms of raw numbers, there are probably many more heterosexual “swingers” than there are gay men in open relationships, yet we still allow heterosexual couples (including swingers!) to marry. On what grounds, then, can we deny marriage to same-sex couples, including those who pledge and achieve sexual exclusivity? The argument works even less well against gay sex than it does against same-sex marriage: The moral status of one person’s sexual acts does not hinge on what other members of his or her sexual orientation do.

In short, the causal version of the PIB arguments fails, both as a prediction and as a moral objection.

Taboos and Moral Reasons

There is still another way of understanding the PIB argument. Maybe there’s no direct logical connection between endorsing homosexuality and endorsing PIB. And maybe the gay-rights movement won’t have much effect, one way or another, on the polygamy movement. Still, the very process of challenging existing sexual mores invites the following concern: If we start questioning some sexual taboos, won’t that make it more likely that we’ll start questioning others? Perhaps some things are best left unquestioned.

If that’s the objection, then much of this piece has missed the point. We’ve been comparing reasons for and against homosexuality with reasons for and against PIB. But some might worry that once we start demanding reasons for established moral claims, morality has already lost its essential majesty and force. This objection is a version of the argument from tradition, but it’s more sophisticated than the simple assertion, “We’ve always done it this way, therefore we should continue doing it this way.” Rather, the idea is that our moral traditions have an internal practical logic to them, even when it’s not apparent on the surface. They evolved the way they did for a reason, and tinkering with them invites peril.

It is worth noting that just because something evolved as it did for a reason, it does not follow that it evolved as it did for a good reason. One of my favorite stories on the subject comes from the late food critic Craig Claiborne. A woman received a ham and was disappointed that she didn’t own a saw. Although she had never cooked a whole ham, she knew that her mother always prepared hams for cooking by sawing off the end, and she assumed it had to be done this way. So she called her mother, who explained that she learned to cook from her mother, who always did it that way -- she had no idea why. The perplexed pair then called Grandma: “Why did you always saw the ends off of hams before roasting them?” they asked her. Surprised, the old woman replied, “Because I never had a roasting pan large enough to hold a whole ham!”

The problem with the argument from tradition, even in its more sophisticated form, is that it takes a good point too far. All else being equal, there’s a good reason to favor “tried and true” practices, and it would be impractical, even foolhardy, for each generation to invent morality from scratch. So I would agree that we should proceed with caution when tampering with long-standing tradition. But it doesn’t follow that we can never revisit moral traditions. Take, for example, the taboo against interracial relationships, which has appeared in many cultures to varying degrees. We can understand how this taboo might have arisen from an overzealous collective instinct for group preservation. There may, indeed, be a further “internal logic” behind it. Yet somehow we also recognize that the taboo causes needless pain and that it ought to be discarded.

My point here is not to suggest a perfect analogy (no analogy is perfect) but to invoke some lessons from history. When a taboo interferes with people’s happiness with no apparent justification, it is probably time to rethink it. Traditions have value, but so too does the process of ongoing moral reflection. We should not confuse reasonable caution with obstinate complacency, which can sometimes be a cover for bigotry. The process is challenging, to be sure, and there are no shortcuts. It’s easy to draw lines around things we don’t like and then condemn others for falling outside the lines; it’s much harder to articulate a coherent, complete and plausible sexual ethics. It’s especially hard to do so when people keep changing the subject -- which, in the end, seems to be the PIB argument’s main function.

Reprinted from "What’s Wrong with Homosexuality?" by John Corvino with permission from Oxford University Press USA. Copyright © 2013 by John Corvino.