If he had donated $1000 to a fund opposing inter-racial marriage, or to a fund opposing voting rights for a class of citizens, would you still feel the same way?
Bravo. This is exactly the argument we should make. If he had donated to pass an anti-miscegenation law, we would all know it was wrong.
You're free to believe whatever you want. You're free to believe men and women marrying men and women is wrong. You're free to believe that whites marrying blacks is wrong. It IS supposed to be a free country.
But when you donate to pass a law about it, it changes from belief to infringement on rights.