Marriage, remember, is not just a contract between two people. It is a contract that two people make, as a couple, with their community – which is why there is always a witness. Two people can't go into a room by themselves and come out legally married. The partners agree to take care of each other so the community doesn't have to. In exchange, the community deems them a family, binding them to each other and to society with a host of legal and social ties.
America needs more marriages, not fewer, and the best way to encourage marriage is to encourage marriage, which is what society does by bringing gay couples inside the tent. A good way to discourage marriage, on the other hand, is to tarnish it as discriminatory in the minds of millions of young Americans.