This reminds me more than a little of reactions to the integration of women into various civic insitutions over the course of the 20th Century--if you're ever in need of a pick-me-up that will put a sly smile on your face, go back to debates preceding the gender integration of any given all-male college or university. One of the more common predictions that tended to crop up in these debates from the defenders of single-sex institutions was that it would lead to a genderless, androgynous climate on campuses (either that or it would lead to plummeting intellectual standards as lust-crazed males became hopelessly distracted by women in their midst).
Ok, so that didn't happen. Is there any reason to imagine it will in this case? I think in part Yglesias' argument is premised on a gloss of Butler's position--while it is true that she argues that the male-female binary is intertwined with the homosexual-heterosexual one, she also argues that this binary has never described the actuality of gender identity, only its dominant representation. We always already have had more than two genders and more than two sexualities, in her view.
And one of the important things to grasp here in particular, not so much from Butler but from the general history of modern sexualities is that there are plenty of forms or genres of homosexual sentiment and practice which actively reify and exaggerate the male-female difference. This is what is so odd about Yglesias' argument, that it is so distanced from the actuality of gay and lesbian life, or from heterosexual life to boot. Go order up some Tom of Finland material from Amazon, Matt!
There's a kind of intellectual gesture that I've come to call"Stupid Foucault Tricks", where you observe that attempts to radically transform certain kinds of institutional systems or fixed binaries often end up absorbed within those systems and reinforcing them. This is a time for one of those gestures. I see no reason to suppose that gay marriage will dissolve existing distinctions of gender or sexuality, and every reason to suppose that it will in fact reinforce both.