The dirty little secret about gay marriage: Most gay couples are not monogamous.We’ve come to accept lately, partly thanks to Liza Mundy’s excellent recent cover story in The Atlantic, and partly because we desperately need something to make the drooping institution of heterosexual marriage seem vibrant again, that gay marriage has something to teach us, that gay couples provide a model for marriages that are more egalitarian, less burdened by the old gender roles that are weighing marriage down these days.But the thorny part of the gay marriage experiment is sex, and more precisely, monogamous sex. Mundy writes about an old study from the ’80s that found gay couples were extremely likely to have had sex outside their relationship — 82 percent did. That was before AIDS and the great matrimony craze in the gay community. She also tells the story of Dan Savage, who started out wanting to be monogamous until he and his partner had kids, and then they loosened up on that, in order to make their union last. “Monogamish” is what he calls his new model. But as Mundy asks, can anyone out there imagine a husband proposing that same deal to his pregnant wife?A long Gawker story last week explored this problem in greater detail. In the fight for marriage equality, the gay rights movement has put forth couples that look like straight ones, together forever, loyal, sharing assets. But what no one wants to talk about is that they don’t necessarily represent the norm.“The Gay Couples Study out of San Francisco State University — which, in following over 500 gay couples over many years is the largest ongoing study of its kind — has found that about half of all couples have sex with someone other than their partner, with their partner knowing,” the Gawker story says (italics in original).In writing about the subject, gay people emphasize the aspects of their relationships that sound most wholesome and straight-like, Steven Thrasher writes. They neglect to mention that, say, in Thrasher’s case, he met his partner for sex only once, and they ended up falling in love. The larger point being that gay couples are very different when it comes to sex, even if this is not the convenient moment to discuss that. And in legalizing gay marriage, we are accepting a form of sanctioned marriage that is not by habit monogamous, and that is inventing all kinds of new models of how to accommodate lust and desire in long-term relationships.In his interviews with married gay couples, Thrasher gets them to open up about the arrangements they invent. Most are some version of Dan Savage’s “monogamish.” They are monogamous when they are in the same city, they can have sex with other people but not fall in love or have sex with others for some period of time. In some far off ideal world, this kind of openness may infect the straight world, and heterosexual couples actually start to tackle the age-old problem of boring monogamous sex. But do any of us really believe that? Hanna Rosin is the author of The End of Men and a senior editor at The Atlantic.