As an aspiring social scientist, this "traditional marriage" argument really gets to me. It makes no sense. AT ALL. There is no such thing as a "traditional marriage", and any SS worth thier salt, and that's what they'll tell you. In history, the most common form of marriage is one man with many women, so if we're going by what happens most often in history as "traditional" then polygamy would be the traditional form of marriage.
There is no one form of marriage that is guaranteed to fail, and no one form that's guaranteed to succeed. Furthermore, no society has ever had a successful marriage system based on the notion of romantic love, which is probably why the divorce rate is so high.
The term "traidional marriage values" is nothing more than a buzzword. It doesn't mean anything, much less what every anti-homosexual politician says it does. The arguments presented for these traditional values and against more progressive ideas flies in the face of real social science. Frankly I don't see how anybody who actually does a small amount of research in the field on this subject could even fathom agreeing with them.