One, gay people deserve to be just as miserable in marriage as straight people. Two, if your marriage is so frail that my marriage to Xi'an would have destroyed it, then you have bigger problems to worry about. Three, marriage is not a Christian institution. Marriage is in fact a legal contract in the United States, and as such should be freely available to any consenting adult, regardless of religion, race or gender.
Fourthly, if you want to pull the bible in allow me to highlight the passages allowing multiple wives, rape-as-marriage women-as-property and all sorts of other things modern society has rejected as barbaric.
Finally, just wait until we can combine two eggs to make a baby, you'll be doomed!