Gays don't want the right to marry, gays want to change the definition of marriage. And, on top of that, the glbt community seems to want to throw out such "old fashioned' things like fidelity and being concerned about the welfare of children.
"If an adult man chooses to embrace homosexuality, that's one thing. But two men imposing their homosexual lifestyle on an innocent, impressionable child-thus intentionally depriving that child of a mother-is something quite different." My supporting info is here.