@appolose,
if I'm not mistaken isn't that Jesus riding the ass in your avatar? If so I would assume you're a christian, which is why I find it strange that you'd use nihilism as a defense of denying gay rights. Not too many nihilistic anti-gay rights christians.
As a largely nihilistic person myself I certainly won't argue about intrinsic moral value. The protection of civilization is not inherently a good thing, the preservation of human life is only good because our replicating genes demand it and isn't in any universally way preferable to the ending of all life. So when I look at social/political/whatever the hell kind of problems I look at it much in the way I'd look at a rubiks' cube, merely a puzzle to be solved. It's not that the colors intrinsically are better off all on one side, but the guy that made the rubik's cube obviously intended that to be the goal.
Civilization as a whole is attempting at moving in a direction that preserves the liberties of each individual without encroaching on the rights of others. The goal of man at present seems to be the equality of all with harm to none. Is that intrinsically better than anarchy, rape, pillaging, goat sex and snail worship? No, but man as a species sees equality and liberty as the best means of replicating genes and preserving itself as a species, which intrinsically right or not certainly seems to be the goal.
So when I say "gays deserve to get married and be treated just like any one else" it is not because I think that is fundementally a "good" thing seeing as how I don't think "good" exists. But I think it is getting all of the red squares on one side as per the goal intended. Your slippery slope argument of "Next is pedophelia" is lame and I think that you know that, you don't seem dumb enough to assume that if you give a person gay rights next is viking attacks or some other disjointed absurdity. But I do get that pedophelia isn't inherently right or wrong and is merely an arbitrary value judgement. However pedophelia goes against the goal of man's civilizations which is to preserve liberty and prevent harm to the individual. As such that would be putting a blue square with the red, and nobody wants that.
So you can argue against objective morality all you want, because I find it really quite irrelevant.