So it’s hard to imagine polygamy being embraced as a major progressive cause or hailed as the next great civil rights movement. (I’m doubtful that most of Gallup’s pro-polygamy 16 percent see it that way now.) And the courts, being political entities, are unlikely to redefine marriage further merely because the logic of past rulings points that way.

On every issue save abortion, social liberalism is suddenly ascendant in America. The shift on same-sex marriage has captured the headlines, but the change is much more comprehensive: In just 15 years, we have gone from being a society divided roughly evenly between progressive and traditionalist visions to a country where social conservatism is countercultural and clearly in retreat.

This reality is laid bare in the latest Gallup social issues survey, which shows that it’s not only support for same-sex marriage that’s climbing swiftly: so is approval of unwed parenthood (45 percent in 2001, 61 percent now), divorce (59 percent then, 71 percent today), and premarital sex (53 percent then, 68 percent now). Approval of physician-assisted suicide is up seven points and support for research that destroys human embryos for research is up 12, pushing both practices toward supermajority support.

Oh, and one more thing: The acceptance of polygamy has more than doubled.

Now admittedly, that last one is an outlier: Support for plural matrimony rose to 16 percent from 7 percent, a swift rise but still a very low number. Polygamy is bobbing forward in social liberalism’s wake, but it’s a long way from being part of the new permissive consensus.

Whether it will eventually get there is an interesting question. Many social conservatives argue that it will — that the now-ascendant model of marriage as a gender-neutral and easily-dissolved romantic contract offers no compelling grounds for limiting the number of people who might wish to marry. And conservatives do have a pretty good track record (the consolation prize of cultural defeat) when it comes to predicting how the logic of expressive individualism unfolds.

At the same time, social change happens sociologically, not just logically, and as a social phenomenon polygamy is very different than same-sex marriage. It’s associated with patriarchy and sexual abuse, rather than liberation and equality. It flourishes in self-segregated communities, Mormon-fundamentalist and Muslim-immigrant, rather than being widely distributed across society. Its practitioners (so far as we know) are considerably fewer in number than the roughly 3.5 percent of Americans who identify as gay or bisexual.

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