Christians don't even have a right to define what marriage is, its different for everyone of different backgrounds, cultures and religions. Its an ancient practice with multiple definitions. America, a country supposed to be known as a melting pot of these things needs to take this into account.
If you believe passages from a book written 3,500 years ago that the church deemed was "The World of God" and people over the centuries have just followed then that is totally your right. But you have absolutely no right to impose these beliefs upon complete strangers with different beliefs.
