What is the history of marriage? No one knows where it originated, but it is something that exists is all societies. Marriage existed pre-government, so when we talk about allowing gay marriage, we are talking about the government construct of marriage. Governments today license marriage, rather than just recognizing it. In Michigan, one must get a license from the state in order to get married. It should be the other way around. Marriage started (in the western tradition) as a religious ceremony. Other, non-christian, societies also recognize marriage, so it's not a strictly religious engagement.
Therefore, government's place should be simply to recognize marriage, regardless of who it is. What right does government have to determine what is and is not acceptable in marriage? What legal or philosophical grounds can anyone rationally present against the right of two consenting adults, regardless of sex, from entering into a binding relationship?