I think I heard it summed up best as follows: "If you oppose gay marriage, don't marry a gay person."

Religion has no place in secular issues. Opposition to gay marriage is by and large, a religious standpoint.* A state sanctione marriage is, by its nature, such an issue. Leave the religion angle out of it entirely, any any opposition anyone has at that point in time should be gone.

*I'm sure it isn't in all cases, but I can't think of any off the top of my head, either.