I think the evidence that gay marriage should be a right is the same as the evidence that "straight" marriage should be a right: the evidence is that it does not infringe on anyone else's rights, and in a free society people should have the legal right to live their life as they choose as long as they aren't infringing on other peoples' rights.
Pedophilia, rape, theft, etc. all infringe on the rights of other citizens. I know you and Victor say you aren't making a comparison between pedophilia and rape and gay marriage, but in fact that is exactly what you are doing. Imagine how flabbergasted you would be if you were talking to someone who opposed interracial marriage, and he/she mentioned the fact that "Hey, we have to have some morality, you know....if we allow interracial marriage, why not allow pedophilia, rape, and theft?"