A nice analogy:
America does not currently conceive of marriage in the way that Douthat and Tushnet would like it to conceive of marriage, and in the way it would need to conceive of marriage in order for there to be a good reason the institution can't accommodate gays. So to oppose gay marriage, Douthat and Tushnet must first construct an alternative version of marriage, and then argue that if real marriage opens to gays, that's another step away from the idealized marriage that would be closed to gays.
It's like partisans of VCRs opposing improvements to DVDs because they make the widespread resurrection of VHS unlikely.
It seems to me that we are witnessing the much faster collapse of the anti-gay marriage case - on logic and public opinion - that almost anyone anticipated. It is as if suddenly, one consensus has imploded and another begun.