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Free To Be You and You and MeHomosexuality, polygamy, and incest.

By William SaletanPosted Friday, May 16, 2008, at 1:40 AM ET

Read Slate's legal bloggers' reactions to the California same-sex marriage ruling on Convictions. Also in Slate, Kenji Yoshino calls the decision "revolutionary," Emily Bazelon explains why voters might not freak out, and Dahlia Lithwick explores which branch of California's government has been most "activist."

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I used to brush off polygamy as an anti-gay scare tactic. But now there's a real connection: The U.S. Supreme Court's 2003 ruling in favor of a right to private homosexual conduct encouraged an emerging détente between Mormon polygamists and state governments. According to the Washington Post, state officials "offered a deal: Marry however often you want, but don't marry children." A spokesman for Utah's attorney general tells the Post, "We're not going to prosecute people solely for adult bigamy."

In other words, polygamy now has the same legal status as homosexuality in most jurisdictions: Your second marriage won't carry any legal weight, but it'll be tolerated.

In reality, most polygamist communities are authoritarian and push girls into marriage before they're old enough. That's why Texas raided a polygamist compound last month. But the raid has actually clarified the distinction between plural and underage marriage. "This is not about polygamy," a Texas government spokesman tells the Dallas Morning News. "It is about child sexual abuse and our commitment to protect children."

Furthermore, the raid—complete with bogus intelligence and an aftermath fiasco—is already doing for polygamy prosecution what the Iraq war has done for invasions: reminding us why it's better to stay out.

Now comes the third item in Santorum's axis of evil: incest.

Is incest unnatural? Not exactly. Last month, Science News reported that inbreeding is surprisingly common in nature, apparently for sound Darwinian reasons.

Is it common among humans? Not as a brother-sister arrangement. But millions of people are doing the next-best thing. In a sample of Pakistanis, first-cousin couples accounted for around 60 percent of all marriages. In a sample of Indians, first-cousin couples accounted for one-third of the marriages, and uncle-niece couples accounted for one-fifth.

Do cousin marriages lead to genetic disease? Generally, no. Six years ago, a study by the National Society of Genetic Counselors found that having a child with your first cousin raised the risk of a significant birth defect from about 3 percent to 4 percent to about 4 percent to 7 percent. The authors concluded that this difference wasn't enough to justify genetic testing of cousin couples, much less bans on cousin marriage.

Is this just a foreign problem? Nope. Charles Darwin, Albert Einstein, and Rudy Giuliani married their cousins. And globalization is bringing Asian practices to the West. In Britain, the challenge is coming from Pakistani immigrants. Next week, the Royal Society of Medicine will discuss genetics in a multiethnic society. A week later, geneticists will hold a forum titled "Cousin Marriage: A Cause for Concern?" Defenders of the practice are ready. In addition to the data about low probability of birth defects, they note that women are increasingly having babies in their 30s, which multiplies the chance of Down syndrome. Why tolerate one risky choice but not the other?

We'd better start thinking about these questions. Because we're going to have to answer them.

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William Saletan is Slate's national correspondent and author of Bearing Right: How Conservatives Won the Abortion War. Follow him on Twitter here.

Photograph of the Tom Green family by AFP/Getty Images.

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