Seriously, that was the question that kept running through my mind while reading Sam's latest essay in the Weekly Standard. I definitely encourage you to read the whole thing, because you really really need to. However, here's the synopsis:
Gay Marriage is a Bad Idea Because:
1) Marriage is fundamentally about deciding who can and cannot have sex with a woman
2) Gay marriage means two brothers could have sex, or that an Italian may marry an Irishman
3) Sex before marriage is bad (which isn't actually an argument against gay marriage)
4) Marriage means you're an adult (which also isn't an argument against gay marriage)
If you think I'm making these up or exagerating, please, by all means, go and read the essay. I'm not. He even helpfully numbers them for you.