I'm making a very narrow argument that marriage was intended to support families, and families come from ordinary mating. My main concern is the legal redefinition of the English word marriage. It seems America's judges and some legislatures too weak to point out that only males and females procreate, and that marriage is basically government involvement in procreation.
Torie has already pointed out the weakness in your argument. Marriage is not only about procreation, and need not even have anything to do with it. I get frustrated when liberals say that marriage is about love and what business is it of the government if two people love each other. Marriage is not just about love.