Maggie Gallagher answers that question in the L.A. Times. The first paragraph below struck me as simple but incredibly profound in the context of the public discourse today. Marriage is something in particular with a definition that transcends our time and culture. Marriage predates our cultural rules. Marriage has a purpose embedded in the physical union of a man and a woman. Marriage has a definition that can't be redefined because we want to because we can't change the nature of reproduction, the fundamental need of society for marriage.
It's simple: Government did not create marriage. Marriage is older than the U.S. Constitution, older even than the Bible or the Koran. Marriage's deepest roots are in human nature and human experience. Marriage, as a judge on the Connecticut Supreme Court wrote in his compelling dissent to that court's recent ruling allowing gays to wed, is rooted "in biology, not bigotry."