TJ, if it is all about the money (and marriage is not all about government subsidy - ever hear about the "marriage penalty" in the tax code?),...
True, there are some logistical benefits of marriage, mainly that the government recognizes you as family. This is still essentially a subsidy since most people view having a legal marriage as on the net beneficial.
That's becuase there are people who feel gay marriage is personally unacceptable but should be legally recognized, perhaps they see some value in a committed relationship regardless of sexuality or because they view "marriage" as something reserved for religious purposes and "civil union" as allowing benefits to be received. I don't see much difference between the two--no matter what you call them--but a lot of people do.