You should really care that the German Ethics Council (a government committee) has pronounced that Germany's laws banning consensual incest between adult brothers and sisters ought to be abolished.

Now, it's not because Germany's laws directly affect the United States; they don't, of course. And even within Germany itself, the ruling party of Chancellor Angela Merkel has indicated that it doesn't plan on following the council's recommendation.

The reason why Americans — and especially American liberals — should care about the council's ruling is that it gives us a glimpse of America's future.

The German council's position is based on the claim that "the fundamental right of adult siblings to sexual self-determination" overrides all other moral considerations, including "the abstract idea of protection of the family." That is very similar to the rationales that have been used to uphold reproductive rights and to strike down bans on same-sex marriage throughout the United States. And we have every reason to think the same logic will eventually apply to bans on sibling incest in this country.

Many liberals will want to deny this, since it sounds like a slippery-slope argument, which social conservatives love to deploy but liberals prefer to mock and dismiss, often illustrated by outrageous and ill-informed quotations from Rick Santorum. But slippery-slope arguments are aren't always foolish. On the contrary, they can be powerful and persuasive when they simply point out that justifying activity A with reference to principle X opens the door to justifying activities B, C, and D, if they, too, conform to principle X. All it takes is someone to make a claim in favor of activity B, C, or D for that person to be granted the same rights as those making a claim in favor of activity A.

This is precisely what Supreme Court Justice Antonin Scalia has been banging on about since his blistering dissent in Lawrence v. Texas, the 2003 decision that overturned Texas's anti-sodomy statute. Focusing less on the details of the Texas law, Scalia was primarily concerned that the libertarian rationale used to strike it down would apply just as well to any law seeking to regulate consensual sexual behavior, including bans on "bigamy, same-sex marriage, adult incest, prostitution, masturbation, adultery, fornication, bestiality, and obscenity."

Scalia made a similar slippery-slope argument in his dissent in United States v. Windsor (2013), the case that struck down the federal Defense of Marriage Act. In Scalia's view, it didn't matter that the majority had refrained from finding "a constitutional requirement to give formal recognition to same-sex marriage." By denying that there is any possible rationale for opposing same-sex marriage besides the desire to "'disparage,' 'injure,' 'degrade,' 'demean,' and 'humiliate' our fellow human beings, our fellow citizens, who are homosexual," the majority had established a principle that rendered both state-level bans on gay marriage indefensible and an eventual declaration of a constitutional right for homosexuals to marry inevitable.

A series of lower courts have subsequently paid backhanded tribute to the power of Scalia's argument by relying in part on his dissent in overturning numerous state-level bans on same-sex marriage. As for his claim about an inevitable constitutional right to gay marriage, it hasn't been declared yet. When will it happen? Since the principle justifying such a right has already been established, Scalia thinks it could happen any time, with the primary consideration being what the majority believes "it can get away with."

Well, how long before the majority wants to and believes it can get away with declaring a constitutional right to sibling incest?

Don't laugh. As with same-sex marriage, the principle has already been established. In a notorious passage of Planned Parenthood v. Casey, the 1992 decision that upheld the constitutional right to abortion, Justice Anthony Kennedy wrote that "at the heart of liberty is the right to define one's own concept of existence, of meaning, of the universe, of the mystery of human life." Kennedy relied on the same passage in his majority opinion in Lawrence. The German Ethics Council expressed a similarly libertarian ideal of sexual autonomy in its ruling in favor of sibling incest.

Perhaps you still think the comparison is silly. But it's not. Here are some relevant questions for liberals. I think you'll be surprised where the answers lead you.

Do you support the right of consenting adult brothers and sisters to marry? If not, why not? What legal or moral principle justifies granting marriage rights to unrelated same-sex couples while denying such rights to brothers and sisters?

Note that the German Ethics Council also held that the prospect of a brother and sister producing children with genetic defects cannot be used as a reason to deny them a right to marry. After all, disabled couples are not prohibited from procreating under German law, even though they have a greater-than-average chance of producing disabled kids. The same is true, incidentally, under the Americans with Disabilities Act of 1990.

If you do support the right of brothers and sisters to marry, is it because you think there's nothing wrong with (or even something potentially good about) incestuous relationships? Or do you support the right to incestuous marriage despite being disgusted by the practice?

This question is significant because in recent years some liberals have begun to argue that it is not enough for traditionalist religious believers merely to tolerate same-sex marriage. Instead, these people must positively affirm the legitimacy and goodness of gay marriage as an institution. Otherwise, they run the risk of perpetuating the evil of homophobia. Which brings us to:

Is it acceptable to affirm the right of incestuous couples to marry while continuing to think that such marriages are depraved? Or should we combat such anti-incestuous beliefs — just like racism, sexism, and homophobia — on the grounds that they will encourage hurtful stereotypes?

I suspect that liberals won't appreciate being asked these questions. But refusing to answer won't stop the logic at work in the sexually libertarian principles that on other occasions liberals triumphantly champion. Once a person, couple, or group of people make a sexual-partnership claim based on autonomy and consent, there is increasingly no basis on which to legally reject it. And once it becomes legally accepted, there is increasingly no basis on which to morally reject it.

Which means that, sooner or later, incest is likely to be legal and morally accepted in the United States.