Mike, you agree that sibling and polygamous marriages have issues and shouldn't be allowed. You see no problem with gay marriage, in a vacuum. Can't we be grown-ups and make grown-up decisions? Allow gay marriage, and recognize the flaws of sibling and polygamous marriages and don't allow them.
Tec, I'm confident gays don't want to get married for the sake of "political correctness." They want to get married because they're in a relationship with someone they love, and want to get married just like anyone else.