One more time. I favor laws about marriage that don't have as their raison d'etre, as a matter of limitation, procreation. Marriage is a legal status, and is what we choose to define it as. If you think invoking natural law and biology, advances your case, fine. It has zero relevance to me, zero. But if you think it is persuasive with others, go for it.
Hopefully, this is going to take us into the meat of your argument on why you think we should move towards gay marriage, based on policy. What is the state's compelling interest in marriage, beyond how it resulted in what was recognized as a positive good for society, a more stable environment to raise future citizens?
Move the definition too far from the way most of traditional religions define it, what would be the interest of any church to recognize state authority in the area of marriage? In practice, many Mormons & Muslims are already doing what I'm talking about. Imagine that kind of practice spreading to the majority in the nation.