First off, marriage is not a fucking christian institution! Christianity is not the first religion to create marriage, they just claim they are. It existed before Catholicism or Christianity were even a thing. Even today there are non-christian/catholic weddings! Tribal and other religions have had marriages too.
Also, it has not always been between one man and one woman. In fact, in many societies and religions it was one man and ten or twenty women. If you want my opinion, having that many significant others is more damaging to marriage then homosexuality will ever be.
Also, not every christian believes homosexuality is wrong and there are a lot of gay/bi/lesbian people who are Christians. Who has the right to deny them this important part of their faith if we do decide to treat marriage as a christian institution? What about all the atheist marriages out there? How do they slip through?
By the way, thanks to all you bigot christian conservatives (no offense to one of you reading this... just sort of general, there are exceptions) for showing us just how much "separation" between church and state is. The government has failed to realize that religion plays an OPTIONAL role in marriage and that they cannot deny these rights to people on a religious basis.