Wow. Where to begin on this one? As a Christian, I've struggled with the whole issue of gay marriage. Initially, I was against it, but I came around to tacitly support it for a couple reasons.
Firstly, marriage is a civil right. And since marriage is a matter of the state, everyone should be entitled to it. It's important to remember that church and state are separate. This being the case, a church doesn't have to marry anyone.
Secondly, as I understand the Bible, the problem isn't with gay marriage or even homosexuality in itself. The problem is with homosexual sex, which the Bible describes as damnable (1 Cor. 6:9,10 AMP).
For political expediency alone, I think President Obama made the right decision coming out in support of gay marriage. He's not perfect, but he's got my support in November.