Perhaps a lot of the confusion in this issue comes from the fact that "Marriage" is not just a social construct, a religious construct, or a legal construct, but is actually all three.
Governments are and should be only interested in the legal construct of marriage, and Caps is right, by defining something different for gays, it becomes a sort of Jim Crow thing. Of course, another option would be, government could just do away with marriage altogether and call all such unions "civil unions", and problem solved.
As far as the social and religious constructs of marriage, regardless of whether the legal definition still includes gay couples, our constitution still gives individuals the legal right not to recognize them socially. You don't have to send flowers to the gay wedding. It also allows churches not to perform the ceremonies or recognize the marriages if they want. The Catholic church isn't supposed to recognize 2nd marriages according to its own dogma, even though legally speaking, shitloads of Catholics legally get divorced and remarried.