The problem for those who assert biblical authority in support of traditional definitions of marriage is that one could with equal validity assert that the lending of money or certain kinds of haircuts are forbidden by God or that slavery and the subjugation of women are authorized by the Lord.
We cannot change our past. We can not change the fact that people act in a certain way. We can not change the inevitable. The only thing we can do is play on the one string we have and that is our attitude.