Some of those defending marriage as the union of one man and one woman argue from the basis of child-rearing. The two parent, male and female, model for a family is most able to raise well-adjusted children. This is true, but it fails to go as deep as it should. Why should children have a mother and father? What's so special about this combination? Well, they should have a male and female role-model. And why is that? What it all boils down to is the widely recognized truth that men and women are different. Many liberals would have us believe that the only difference between men and women is social, cultural, and some minor physical differences. Psychological studies show that there's more than that; that, in fact, men and women do think differently. They see the world in different ways. They interact with it differently, solve problems differently.
In fact, when God married Adam and Eve in Genesis 2, it wasn't solely so they could have children. Marriage makes two people one, not just in some metaphysical sense, but as a social unit which has a more complete understanding of the world than either one has alone, and who face it as a united front. A couple of two men or two women is incomplete. A single person, such as myself, is incomplete. There's not much that can be done socially or legally to change this reality.