Excellent post. Andrew Sullivan is really being his deep down, conservative self on this issue. And he'll tell you that. Traditional conservatives are freaking out, saying that gay marriage is all about society giving moral sanction to homosexuality. Sullivan just has the opposite evaluation of the same fact. Both are (or should be) wrong. It's about gay spouses getting the same legal rights as straight ones. Period.
It should also be said, though, that Sullivan and the conservatives are hardly alone in seeing the state as conferring legitimacy on gay people. While they arrive at it differently, most people of all political stripes, gay or straight, see it that way. (Hear some of the couples in Mass. being interviewed about how they feel legitimate now?). Which is why it was so nice to read your post. :-)
Thank you, Mark. Like you, I think the practical matters of making a household and taking responsibility for each other should be front and center; that's where the equal protection argument carries weight.
In small doses, appealing to people's sympathies can be very effective. In fact, I'd say it's necessary for gay men in particular, since the Disco Era image that we're all sex machines incapable of love and intimacy and settling down persists in a lot of quarters. But emotional appeals can't be anything close to the linchpin of a public policy argument. That's why I found myself being disappointed with the whole of Rauch's new book while being very impressed, as usual, with the fineness and eloquence of the individual paragraphs and sub-chapters. Sullivan, sadly, started to lose me a while ago.