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.

And while some polygamists may feel they were “born this way,” their basic sexual orientation is accommodated under existing marriage law even if the breadth of their affections isn’t, which makes them less sympathetic than same-sex couples even if their legal arguments sound similar.

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.

With all this said, however, polygamy has already become more mainstream than even a slippery-sloper like myself once expected. The suburban plural marriage on HBO’s “Big Love” seemed like a fantasia when the show first aired, but thanks to the magic of reality television (which has produced three polygamist-themed shows in the last five years) we know not only that such families exist, but that their lives can be turned into bourgeois-seeming sitcom fodder as easily as any other arrangement.

We know, as well, that a bourgeois polygamy can win victories in federal court, as the Brown family of “Sister Wives” fame has done: Not formal recognition for their marriage(s), but the right to live as man and wife and wife and wife without fear of prosecution.

And we also know that “polygamy” is just the uncool, biblical-sounding term of art. Call it polyamory or “ethical nonmonogamy” and suddenly you have a less disreputable demographic interested — not only the commune-and-granola set, but the young and fashionable in Silicon Valley, where it’s just another experiment in digital-age social life.

So polygamists don’t have to win explicit marriage rights to become more legally secure, more imitated, less frowned-upon and judged. Indeed, greater acceptance is almost guaranteed.

The question is, what then? Can Americans say a permanent “no” to recognizing plural marriage once we’ve rooted for the Browns to get a “My Sisterwife’s Closet” jewelry line off the ground? Can a cultural left that believes in proliferating gender identities and Bruce Jenner’s essential womanhood draw the line, long-term, when a lesbian couple wants to include their baby’s biological father in their legal family, or when the child of polygamists stands up in court to say he wants his dad recognized as his mother’s legal spouse? Is a culture where prominent men routinely have multiple kids with multiple wives across multiple decades going to permanently deny marriage rights to people who want the same thing, except all at once?

As I said, it’s an interesting question. I feel safe predicting that polygamy will not be legally recognized, with fanfare and trumpets, in 2025.

But it might be recognized in 2040, with a shrug.