Marriage evolution

I wish I knew alot more about the historical evolution of marriage. Anybody know any good books about it? Its my belief that the original societal purpose of marriage is to legally bind a couple for the allowance of childbearing.
There used to be a word called "bastard" that was taken extremely seriously even though some of history's greatest figures were bastards like William the Conqueror. I think the main reason for marrriage was to keep track of lineage for a multitude of purposes, especially to prevent inbreeding. Since gay couples do not biologically produce children with each other this fundametal purpose of marriage is irrelevant. Adoption does not have the same biological imperative. Also marriage in our society is an inherently patriarchal institution with the implicit gender terms husband and wife. If I was gay I would not want to have any part of the patriarchy issues complicating the union. There would be the inherent ego struggle of the label husband and wife which could endanger the relationship. "Hello, this is my WIFE Thomas". Even if I was gay anybody that called me or thought of me as his wife would be dead. Civil union is much simpler and more efficient all the way around.