The doctrine of marriage depends on Genesis being true. If there's an absolute authority, and if God's the Creator, He made one man and one woman. Jesus came and said that marriage is between a man and woman. If Genesis is not true, we're just animals, and marriage is just whatever you want to make it to be.
If you look only as Genesis as an allegory, you have a major problem, because if it's an allegory, then tell me who our ancestor was? If Abraham was real, then from Abraham if Adam isn't real, if it's just an allegory, it's just a story, then what's the real Adam who really fell in a garden and really sinned? Where did we come from?
There's been an incredible censorship in America and throughout the world, but particularly in America where students aren't even allowed to critically think about evolution, the issue of origins; they are not allowed to hear other points of view; they are taught incorrectly about science and taught that evolution is fact.