Incest is for hicks. That’s the stereotype among educated liberals: Homosexuality is urbane, polygamy is for Mormons, and incest is for hayseeds. So when David Epstein, a Columbia University political scientist, was charged last week with third-degree incest for allegedly shagging his adult daughter, the blogosphere erupted. Conservatives called it another sign of moral chaos. Liberals said it was gross but shouldn’t be prosecuted. One side defends the privacy of all consensual sex; the other side sees an inexorable descent from homosexuality to incest.

Let’s try to come up with something better. If gay sex is OK, how can incest be wrong?

The old answer was genetics. Germany’s high court relied on that argument two years ago when it upheld the conviction of Patrick Stuebing for sex with his sister. Of the four children the couple produced, three had physical or mental disabilities. In general, studies show a significantly higher rate of birth defects in offspring of incestuous couples. The reason is simple: Every family has genetic flaws, and if you reproduce within your family, you’re more likely to get two copies of the flaw—thereby producing the defective trait—instead of acquiring a new, protective allele from another family.

Many incest laws in the United States invoke this concept. In patently eugenic language, they forbid sex between “consanguineous” (blood-related) partners. But this rationale won’t withstand close scrutiny or the march of technology. If genetics is the issue, just get a vasectomy. Then you can bang your sister all you want. Or skip the vasectomy and bang your brother. Gay sex can’t make a baby, so the problem is solved. As the German court noted, Stuebing could have dodged Germany’s incest law in precisely this way.

Epstein has been charged under a different law. It prohibits sex with any close relative, “whether through marriage or not.” It also applies not just to “sexual intercourse” but also to “oral sexual conduct or anal sexual conduct.” If the law were rationally based on genetics, it would ignore sex acts that can’t make babies, and it would distinguish relatives by blood from relatives by marriage.

What about the Scottish woman who was sentenced to probation—and remains under threat of further prosecution—for sex with her half-brother? She was sterilized years ago. You can’t prosecute her based on a risk of birth defects.

So let’s set aside genetics and consider the next question: exploitation. Nowadays, when we talk about incest, we tend to think of child sexual abuse. That’s how we use the term in the repressed-memory debate and in abortion legislation. When politicians such as President Obama make exceptions in abortion laws for “rape and incest,” they’re using the terms synonymously, except that in the incest scenario, the rapist is your dad.

But you can’t prosecute Epstein under that theory. According to news reports, his daughter is 24, and their affair began in 2006. That makes her an adult. Furthermore, police say the sex appears to have been consensual. Four years ago, Ohio’s Supreme Court upheld the incest conviction of Paul Lowe, a former sheriff’s deputy, for what the court called “consensual sex with his 22-year-old stepdaughter.” And last month, a 27-year-old Florida woman was sentenced to five years of probation for sex with her father. Clearly, we’re prosecuting people for incest regardless of age or consent.

At this point, liberals tend to throw up their hands. If both parties are consenting adults and the genetic rationale is bogus, why should the law get involved? Incest may seem icky, but that’s what people said about homosexuality, too. It’s all private conduct. To which conservatives reply: We told you so. We warned you that if laws against homosexuality were struck down, laws against polygamy and incest would follow. And now you’re proving us right.

The conservative view is that all sexual deviance—homosexuality, polyamory, adultery, bestiality, incest—violates the natural order. Families depend on moral structure: Mom, Dad, kids. When you confound that structure—when Dad sleeps with a man, Dad sleeps with another woman, or Mom sleeps with Grandpa—the family falls apart. Kids need clear roles and relationships. Without this, they get disoriented. Mess with the family, and you mess up the kids.

That’s the basis on which the Ohio Supreme Court upheld Lowe’s conviction: “A sexual relationship between a parent and child or a stepparent and stepchild is especially destructive to the family unit.” This destructive effect, the court reasoned, occurs even if the sex is adult and consensual, since “parents do not cease being parents … when their minor child reaches the age of majority.” The German court offered a similar argument against sibling incest. Roughly translated, the opinion’s key passage says:

Incestuous connections lead to an overlap of family relationships and social roles and thus to a disturbance of a family bereft of [clear] assignments. … Children of an incestuous relationship have great difficulty finding their place in the family structure and building relationships of trust with their next caregivers. The vital function of the family for the human community … is crucially disturbed if its ordered structure is shaken by incestuous relations.

Liberals tend to recoil from such arguments. They fear that a movement to preserve the “family unit” would roll back equal rights for homosexuals. But that doesn’t follow. Morally, the family-structure argument captures our central intuition about incest: It confuses relationships. Constitutionally, this argument provides a rational basis for laws against incest. But it doesn’t provide a rational basis for laws against homosexuality. In fact, it supports the case for same-sex marriage.

When a young man falls in love with another man, no family is destroyed. Homosexuality is largely immutable, as the chronic failure of “ex-gay” ministries attests. So if you forbid sex between these two men, neither of them is likely to form a happy, faithful heterosexual family. The best way to help them form a stable family is to encourage them to marry each other.

Incest spectacularly flunks this test. By definition, it occurs within an already existing family. So it offers no benefit in terms of family formation. On the contrary, it injects a notoriously incendiary dynamic—sexual tension—into the mix. Think of all the opposite-sex friendships you and your friends have cumulatively destroyed by “crossing the line.” Now imagine doing that to your family. That’s what incest does. Don’t take my word for it. Read The Kiss. Or the sad threads on pro-incest message boards. Or what Woody Allen’s son says about his dad: “He’s my father married to my sister. That makes me his son and his brother-in-law. That is such a moral transgression. I cannot see him. I cannot have a relationship with my father …”

Homosexuality is an orientation. Incest isn’t. If the law bans gay sex, a lesbian can’t have a sex life. But if you’re hot for your sister, and the law says you can’t sleep with her, you have billions of other options. Get out of your house, for God’s sake. You’ll find somebody to love without incinerating your family. And don’t tell me you’re just adding a second kind of love to your relationship. That’s like adding a second kind of life to your body. When a second kind of life grows in your body, we call it cancer. That’s what incest is: cancer of the family.

I wouldn’t prosecute David Epstein. It isn’t necessary. The incest taboo is strong enough to withstand the occasional reckless fool, and I don’t want cops poking around in people’s sex lives. But incest is wrong. There’s a rational basis to forbid it. And we shouldn’t be afraid to say so.

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