Again, exactly who wrote the first 11 chapters of Genesis and when? Predating the Bible is one thing. Predating the Biblical beliefs and ideals handed down generation after generation before Moses is something you cannot define. You can say it over and over, but it doesn't change that you cannot define how old nor even where those beliefs and ideals came from.
I never said that Christian doctrine is considered truth by all American. I don't expect them to. I also expect the state to stay out of religious beliefs and moralities. That is not going to happen.
By the government defining marriage you will create a situation where all churches will be forced to marry anyone/everyone coming in the door at some point. To say otherwise means you haven't been paying attention. I'd rather government stick to unions, and let you get married in the church of your choice, even if that is a liberal church "marrying" a gay couple.