Corrupting [marriage] into something purely based on decadent sex is not wise. For anyone.
But that's generally not what people who are gay are doing. I know two women that raised two or three kids together. Their marriage would not have been "just" about sex. Their marriage would have been a statement to the world that this is the person that comes first for me, which is pretty much the same statement that my hetero marriage does.
But, based on your statement, what about hetero marriages where the couple find they are infertile? At that point is their marriage "purely based on decadent sex"? If so, should we force their marriage to be annulled if they find they can't have children? What if they get married past their childbearing years?
If [one of] the purposes of marriage is to declare monogamy towards one other person, why shouldn't gays be able to publicly declare such a thing? Wouldn't encouraging gay marriage be a means of reducing promiscuous sex?
Honestly, go through and write down all of the reasons that marriage (of any form) should be recognized and then objectively ask yourself if that reason would also apply to a gay couple. Keep in mind that with artificial insemination and adoption, gay couples can be parents. If you get over your own revolution to the gay lifestyle, you might find that your objections to gay marriage start to fall away.