Marriage as a worldwide phenomenon predates Christianity. Early Christians considered marriage to be a social convention that required neither religious or civil ceremony. There have been same sex unions around the world for as long as human history has been recorded. Long before Christianity existed.
Are you referring to the US as a Christian nation? That's highly debatable considering it was founded by Diests who believed in freedom of religion. I suggest that rather than telling people to move away if they don't like Christianity that you move away if you can't be tolerant of non Christian points of view.