I'm sorry, but I just had to laugh when I read Robert George's description of the Catholic Church's position on gay marriage: namely, that the church "opposes the re-definition of marriage to eliminate the requirement of sexual complementarity."
Not only is the guy reaching hard to euphemize (which is a good sign, as it signals that the bare language of "excluding gay people" isn't tolerable anymore), but it is virtually impossible to parse. I think the intuitive definition of being sexually complementary is sharing a mutual attraction and compatible desires vis-a-vis giving and receiving sexual pleasure. But here, it looks like that is being replaced with juvenile "tab A in slot B" or "entrance, not exit!" argumentation. It is testament to the increased potency of the pro-equality argument that opponents now have to work so hard to obfuscate what they're actually doing.
It's really simple. Right now, marriage is open only to heterosexual couples. Homosexual couples are excluded, for no real reason other than irrational prejudice and/or animus. That's incompatible with American norms of equality. And eventually, it will fall.