1) Gay marriage is incompatible with Christianity (and for that matter, Islam & Judaism). If someone asks you why you oppose gay marriage, the only thing you really have to say to explain it is, "I'm a Christian."
God doesn't condemn anyone for who he is; so if you're attracted to the same sex, that absolutely, unconditionally doesn't make you bad, evil or "un-Christian." On the other hand, let me note that I do consider hating, tormenting, or bullying people because of their sexual orientation to be distinctively "un-Christian" behavior. As Billy Graham has said, “God will not judge a Christian guilty for his or her involuntary feelings.” However, God has drawn a clear line in the sand when it comes to homosexual acts. If you're gay, you're not allowed to act on it. If that seems harsh or unfair to you, well, sorry, but you'll have to take it up with God. It's His rule.
"Thou shalt not lie with mankind, as with womankind: it is an abomination." -- Leviticus 18:22
The people of Sodom and Gomorrah could tell you how serious God is about that -- if there were any of them left. So as a Christian, you can no more condone gay marriage than you could give the thumbs up to prostitution or wife swapping. That means if you're a Christian standing in favor of gay marriage, then you're a Christian who's standing directly in opposition to the God whom you claim to worship.
2) Gay marriage will end up infringing on religious freedom. The moment gay marriage becomes the law of the land, all sorts of First Amendment freedoms involving the free exercise of people's religion will likely be infringed upon as a consequence. No pastor should be forced to marry a gay couple. No wedding photographer, cake maker, caterer, or wedding planner should be forced to be involved in these weddings. No church or any other location should be forced to be the site of a gay wedding. Children will be taught in schools that gay marriage is normal, legal, and moral -- and it directly contradicts the teachings of Christianity, Judaism, and Islam. To create this special privilege for gay Americans would mean impinging on the First Amendment rights of more than 200 million Americans.