I think it comes from a moral understanding today that gay people are no different, and that gay married couples' relationships are not significantly different from the relationships of straight married people.
Look, I'm not for discriminating against anyone in their employment, or housing, or ability to get benefits from the federal government (except to the extent that we can't afford benefits for anyone anymore). But can we stop pretending that things that are obviously different are actually the same? I mean, seriously... the level of euphemism and obfuscation with regard to talking about these issues is getting to be insulting to anyone of average intelligence.
And, by the way, when did gay Americans suddenly start standing for the proposition that the gay lifestyle is really just like Ozzie and Harriett? I had no idea there was such an impulse to conformity on the gay left. Or is it just cognitive dissonance? For decades we've been told that we have to respect diversity. Now it seems like we're supposed to act as if everyone is really just the same... just harmless fuzzballs looking for love and a shoulder to cry on.
Actually, when you put it that way, maybe we are pretty much all like that. Hmmmmm....