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

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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Tom Green's family includes five wives and 29 children (not all pictured)It's that time again: an election year, voters unhappy with the economy, Democrats poised to reclaim the White House. Time for a liberal state Supreme Court to strike down a law against gay marriage and piss people off. In 2004, it was Massachusetts. Now it's California. Democrats are terrified that the ruling will energize the right and turn the election into a referendum on same-sex unions.

Actually, it could get a lot worse.

Gay marriage isn't as politically lethal it used to be. Most voters (including me) now support marriage or civil unions for same-sex couples, and the percentage favoring marriage has increased significantly over time. Conservatives need to push the debate into kinkier territory.

How? Look at the campaign materials of the groups pushing to constitutionalize California's gay-marriage ban. "Establishing same-sex 'marriage' as a fundamental right will undermine current polygamy laws and create a new legal precedent for 'anything goes' forms of marriage," says a talking-points summary. A flier says liberals want "legal recognition for any combination of relationships involving two, three or more people." A memo on focus-grouped messages advises conservatives to ask: "How do we say 'no' to a woman who wants to become the third wife of a polygamist?"

We've heard this slippery-slope argument before. Five years ago, Republican Sen. Rick Santorum of Pennsylvania put it this way: "If 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. …"

I hate to say it, but things are playing out pretty much as Santorum predicted.

It's not true that there's been any cultural rush toward these practices. That was always hogwash, since heterosexuality, jealousy, and aversion to immediate-family incest are broadly grounded in human biology. What's true is that our categorical bans on polygamy and incest, like our bans on homosexuality, are losing their justification.

Two trends are driving this erosion. One is the rise of privacy as a cultural, political, and legal principle. Societies are becoming less able and less willing to forcibly restrict sexual choices. Do what you want, as long as you're not hurting anybody. No harm, no foul.

Second, the assumed harm of taboo sex practices is being questioned and subjected to scrutiny. And the evidence is looking pretty weak.

First came the studies of gay parenthood. A year and a half ago, Mary Cheney, the vice president's daughter, announced that she was pregnant and that she and her lesbian partner would raise the child. Conservatives protested, arguing that gay parents are bad for kids. But dozens of studies compiled by the American Psychological Association showed otherwise.

If you analyze the parenting data that've been presented by opponents of gay marriage, they actually indicate that men, not homosexuals, are the problem group. Men are overwhelmingly responsible for crime, domestic violence, and sexual abuse. In fact, in some studies, lesbians score better than straight parents on affection, active caretaking, and parenting skills. It isn't because they're lesbians. It's because two moms are better than one.

Which brings us to polygamy. If two moms are better, how about three or four? I'm skeptical, since jealousy is pretty hard to overcome. But if three women can get along that way, are their kids really worse off?

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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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