States have no more legitimate authority to define marriage than the courts do. Marriage is what God defined it to be, in the very beginning.
I seem to recall a lot of polygamy in the Bible. Does that comport with your view of "what God defined it to be"?
I don't think that is a standard that we can craft our laws on. The founders could probably have created a theocracy, but their experience led them in a different direction.
I don’t personally disagree with you and I think gay marriage is totally ridiculous. However since government in our real world does get involved in these things it should be up to each individual state as to who they issue a marriage license to for civil marriages.