This blogger has a great analysis of the categories used by each side of the gay-marriage debate. You’ll need to read the whole article for the full effect, but here’s a quote from the middle that is probably self-explanatory.
Thus when a conservative hears someone saying that gay sex is okay, they’re seeing that person dip into the box labeled “What God forbids,” and thus they wonder how, if you can dip into that box for one thing, that’s any different from taking something else out of that same box, like pedophilia or bestiality. But for the progressive, pedophilia and bestiality aren’t in the same box as gay sex. So when the conservative asks how someone can accept gay sex but still condemn pedophilia or bestiality, the progressive says, “What? Where in the world is that question coming from?”