Al Gore describes why he’s in favour of gay marriage, and his explanation pretty much demonstrates the double standards of conservatives—they’re opposed (understandably, I suppose) to any kind of promiscuity, and in favour of marriage and commitment and "family values" and so on, but also opposed to allowing a sizable minority of the population to marry; that’s obviously not going to promote promiscuity, but neither is it doing anything to help their other goals.
"Shouldn’t we be promoting the kind of faithfulness and loyalty to one partner regardless of sexual orientation? Because if we don’t do that, then to that extent you are promoting promiscuity and promoting all the problems that can result from promiscuity."
On a similar note: an article on why "family values" doesn’t necessarily mean heterosexuality.