Who gives a damn about nature, if I may ask?
Technically, gay marriage is not more unnatural than using contraceptives or eating a steak with bordelaise sauce instead of biting a cow as it walks by.
Gay marriage "beats the purpose" of sex? Ok, so do contraceptives (and catholic priests, but that's a different story altogether). And when I eat, I rarely do it with the sole purpose of not starving to death: I do it because I enjoy it. Many times I eat when I'm not hungry and drink when I'm not thirsty just because I like it.
A fine painting or the works of Beethoven or Shakespeare have no "natural purpose" whatsoever and I still love them.
While all animals are capable of this, humans are remarkably good at turning things into more of an art and less of an instinct. That's why nutrition became gastronomy and perfectly good mammoth hides became pretty awesome leather jackets, and that's why a crazy German can devote his entire life to writing musical notes on a sheet of paper just to make my evenings more interesting.
And hey, sexy lingerie won't make my offspring grow stronger and healthier, but what the heck, I love it when my girlfriend wears it.
As for the institution of marriage, I have nothing against it, but it has changed a tad. Like safe-keeper said, women's rights have had a huge impact in it, as one of its original purposes was keeping women tied to the household and keeping them from fleeing when their husbands spent a fortune in submarine simulators and naval miniatures
