"For instance, in the debate over gay marriage, one commentator after another likens arguments against same-sex marriage to arguments against interracial marriage."
Jonah, you've got to give the devil his (her? - one mustn't be sexist!) due. This amounts to a *masterful* double-whammy leftist cliche. In one collective (is there no other kind for the left?) breath, the progressive commentariat gets to: A) question the motives of the dissenters, and B) point out that, yet again, the dissenters are on the wrong side of history. Ta-da!
"The Tyranny of Cliches: How Liberals Cheat in the War of Ideas," by Jonah Goldberg. It's a great book, Jonah. You ought to read it. ;-)