Marriage does not (at least not directly ).
Perhaps I should clarify my statement: "What I AM against is the creation of laws to regulate something many persons consider part of their religion. Unless, of course, said something causes harm to others without their consent."
I would not take much issue, personally, if gay marriage were made legal in all 50 states. I might not prefer it, due to lingering training in my formative years, but it wouldn't really bother me all that much.
However, as I view the institution of marriage as a religious one, I have issue with regulations upon it being made by civil governments. Barring, of course, indirect ones, such as those preventing people from performing human sacrifice during a marriage ceremony. (I assume there are such).