I've always felt that the strongest argument against "gay marriage" is to eliminate any legal references to marriage at all. My rationale is that the government has a well-documented track record of fouling up just about everything it gets its hands on, and marriage has been no different. This is why you have entire subsectors of the legal profession dealing with the numerous cases when "legal" marriages fall apart. If contract law, real estate law, patent law, etc. had that kind of a track record, we'd still be living in the Stone Age.
What I do know is that I don't give a flaming sh!t about how the U.S. government or any of its increasingly irrelevant institutions define marriage -- for better or for worse.