I think people are still getting a bit too caught up in what the definition of marriage should be. Talk about unification of souls and the blessing of children all you like, it has nothing, nothing to do with the legal definition of marriage.
Marriage, for the purposes of this discussion, is a legal contract. It is precisely what we make it. It makes just as much sense to say "marriage is between a man and a woman" as it once did to say "land ownership is for whites" or "voting is for males". The structure and wording of these laws change to suit the times. This one will change too, it's just a matter of when.