I've seen this argument rear its ugly head with people close to me. The real problem in my family is that in the early 80's my aunt and uncle moved out of the Chicago area, when we were all still up there, and my cousins were still very young.
They moved into the Tampa/St. Pete's area, it didn't seem to take long before they fell into this crazy church. I mean, this place got up into everyone's business like flies on dung, even getting into it when my aunt and uncle had marital issues (they've long since divorced and I do put part of the blame on these do-rights sticking their nose into it).
I mean these people were whackos...I got my cousin into tabletop D&D not long before they moved, and of course the church decided that was evil as well. My aunt and uncle ended up confiscating his books. It's the same thing later, with groups that want to associate Harry Potter with witchcraft, etc...it's always the same story. Try to identify an evil that wrecking society, all the while missing the fact that YOU ARE the evil.
One of my cousins...well, this the most red-faced and livid I've gotten with any of my extended family, argued passionately against gay marriage, because in her words it violates or destroys the sanctity of marriage. Note that she didn't go and outright call it a sin, but somehow, inexplicably, it messes up 'normal' marriage.
OK, so...hypothetically, you and your significant other are having a perfectly workable and happy marriage and heterosexual relationship. Then one or more gay or lesbian couples move into the neighborhood...do they give off some sort of gay radiation that goes through the walls of your house and ruins your marriage? Seriously.
I have heard plenty of excuses from the opponents, most of time that it's viewed as a sin in the eyes of their faith, and that the alleged offenders are going to hell, or select the disagreeable afterlife of your choosing.
As yet I have not heard a single rational and reasoned argument from the right on this; they're thinking with thier bibles and not their brains, and that's a bad combination. Let me sum it up here-
1. Not everyone goes to your church, you pompous, intolerant prick. Frack you. Some of us take your church as reasonable advice at best, an overblown opinion at worst. Some of us just don't give a shit what you think, and we expect you respect that.
2. Any argument that homosexual marriages somehow taints your own marriages is all your head. This is based on a theological conclusion, on an abstract concept that can never be proven. It's about as fruitful as passing a national proposition to officially recognize the existence of Santa Claus.
That's the thing about this election; we took a big step forwards in equality by electing a black man to our highest office, and then we promptly stepped back on another front by promoting intolerance and bigotry as state law. Although we still have ways to go, racial equality has stepped up dramatically from the civil rights era. In a half century or so we went from a man of color not being allowed to go into the same restaurant as whites, to now walking through the door of the Oval Office.
And that's good, and it means while we still have work to do on that front, racial equality is not the pressing civil rights issue of our current time.
But sexual equality most definitely is.