Maybe someone has mentioned this during this thread, but I missed it because, after reading this thread for a while, I decided I didn't want to read it any longer.
I am a Christian, and I think the Christians who protest gay marriage are wrong. There is nothing in the Bible against gay marriage...nothing at all. There are several verses addressing the act of homosexual sex, but no verses in which God or Jesus condemns the joining in marriage of two women or two men.
I think those Christians opposed to gay marriage should instead use their energy to protest against the homosexual act. If they wait until the marriage ceremony to protest, they have likely missed several occurrences of what they consider sin.
But wait! Don't sinners marry all the time? Gossipers, liars, gluttonous people...they all marry, and there is no hue and cry from the Christian community about those people getting married, even though those sins are mentioned in the Bible. (Some ministers do, however, refuse to marry those they know to be actively, unrepentantly, committing some sort of sin.) Protesting the act of homosexual sex, to me, seems more in line with the verses often quoted from the Bible than protesting gay marriage.
It also seems to me this issue is similar to common "pro-choice" abortion arguments—there is a lack of understanding about the actual issue. The issue, I think, is that pro-life Christians get this main argument from pro-choice people: "It's the woman's right to choose!"
But those who argue this are missing the point entirely. They need to focus on convincing the pro-life Christians that life does NOT begin at conception, and, thus, abortion is not murder. Sure, anyone walking down the street can "choose" to murder someone, but everyone says that's wrong. If ending the life of a fetus/embryo/baby while inside the womb is not murder, then Christians, I assume, would be quite all right with a woman's right to choose. (And I'm not even going to get into being against abortion but being for capital punishment.)
Note I have not stated my personal opinions on gay marriage, homosexual sex, or abortion. I am stating what I have observed about the arguments and disagreements that I see occur in this forum, on TV, and during conversations.