ANTHONY KENNEDY, in his majority ruling legalising same-sex marriage nationwide, tried to allay the concern that polygamy would be next. John Roberts, in his dissent, said he couldn't see a principled way of opening the door to same-sex couples without also letting polygamists through. Who's right? Is legal polygamy next? Should it be? Jonathan Rauch, who wrote this newspaper's ahead-of-the-curve 1996 cover piece advocating same-sex marriage, has plenty of experience in keeping people off this slippery slope. According to Mr Rauch, the problem with a man marrying two women (we don't see many real-world examples of polyandry, a woman with multiple husbands) is that it leaves another, usually lower-status man without a match. As higher-status men hoard wives, lower-status men are "denied access to life's most stabilizing and civilizing institution," he writes, and thus "are unfairly disadvantaged and often turn to behaviors like crime and violence". It's bad for women, too, Mr Rauch says, "because it places them in competition with other wives and can reduce them all to satellites of the man". The abolition of polygamy "democratised the opportunity to marry", which, Mr Rauch suggests, was "a step without which modern liberal democracy and egalitarian social structures might have been impossible." Countries in which polygamy remains common, such as Saudi Arabia, Nigeria and Malaysia, are unusually illiberal, patriarchal, unequal, unhealthy and violent, he argues.

Steven Macedo, a political theorist at Princeton, takes a similar line. Plural marriage, he observes "is strongly associated in practice with patriarchy and class and status hierarchies", while "[t]he spread of monogamy is part of and parcel of the unfinished advance of gender equality around the globe." Mr Macedo allows that adults ought to be free to live as they like, as long as they're not harming anyone. However, "a sober assessment of polygamy as lived social form provides strong grounds for not extending equal recognition to plural marriages."

I am not convinced. Both Messrs Rauch and Macedo celebrate the way monogamous heterosexual marriage has transformed from an oppressive patriarchal institution into an equal partnership suitable for same-sex couples. Given this evolution, it stands to reason that if plural marriage were recognised in America today, it would be subject to the same enlightening forces and cost-benefit analyses. I've heard more than one liberal-minded, double-income couple express a desire for a "wife" who would be happy taking care of the kids and keeping house in return for loving companionship and financial security. Some men might consider sharing a remarkable wife, who wouldn't mind dividing the husbandly labour among multiple spouses. Yet Messrs Rauch and Macedo assume, without bringing forth evidence, that the patriarchal, inegalitarian aspects of historical plural marriage are essential to its nature and incapable of being fixed.

Let's concede, for the sake of argument, that polygamous unions, unlike monogamous unions, are irremediably inegalitarian. In that case, plural marriage won't be very popular in liberal societies. This means that the social pathologies that arise when high-status men "hoard" wives are unlikely to emerge, as low-status men are disadvantaged only when many women see the value of a plural marriage.

It may be true that the abolition of polygamy was a necessary step toward liberal egalitarian societies. But it simply doesn't follow that legalising polygamy now, among liberal egalitarian Americans, threatens to turn back the clock. It may be that smoking bans in bars and restaurants have been absolutely vital to reducing tobacco addiction and smoking deaths. But if those bans are rescinded 100 years from now, how much demand will there be for smoking in bars and restaurants? And how much of a public-health problem would there be if a few people legally smoked in a hazy pub somewhere?

That said, let's suppose, as seems plausible, that the plural marriages that would come about in a more fully libertarian future end up reinforcing inegalitarian gender norms. Well, if that's a reason to disallow them, it's also a reason to disallow many inegalitarian monogamous marriages. In a lucid and illuminating post, Chris Freiman, a philosophy professor at William and Mary, offers a useful thought experiment:

[I]magine that a new club pops up: the Society for Traditional Gender Hierarchies (STGH). Thousands of men and women nationwide sign up and pledge their commitment to the notion that a wife must always be the primary caregiver and a husband must always be the primary breadwinner. Needless to say, this is a bad club. You shouldn't join and, indeed, you should morally oppose the principles of the club. However, even if marriages between STGH members are very likely to reinforce inegalitarian gender norms, they would still receive legal recognition.

Of course, many actual American marriages are, in effect, STGH marriages. But we don't prohibit marriages between those who have adopted traditional gender roles. "Part of liberalism is tolerating illiberality," Mr Freiman rightly says. In the absence of credible evidence that plural marriage in America today would be any more inegalitarian or socially harmful than the old-fashioned patriarchal monogamous marriages that millions of Americans already have, it's hard to justify, at least on liberal grounds, our legal prohibition against more than two consenting adults freely entering into a marital arrangement. As I've argued before, many of the unseemly and unhealthy aspects of existing American polygamous "marriages" are a side-effect of our having made them illegal, and a target for disgust and contempt, much as homosexuality was within living memory.

Perhaps there are other, excellent arguments against legalising plural marriage. But for now, not even extremely sophisticated liberals are making them. Messrs Rauch and Macedo's claims about the harms that would ensue from legalising plural marriage have the same speculative character as some conservative arguments against legal gay marriage. This ought to pique some concern.

Fredrik deBoer, writing in Politico, speculates that liberal opponents of plural marriage remain "trapped ... in prior opposition that they voiced from a standpoint of political pragmatism in order to advance the cause of gay marriage". This is probably right. Now that gay marriage is finally legal from sea to shining sea, it's time for liberals to refine their arguments against polygamy. We need better, more rationally compelling arguments if we wish to be fair in shutting against would-be polygamists the libertarian door that we've just blasted open.