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.
Marriage is a virtually universal human social institution with a certain recognizable shape: It is a public union, not just a private union; it's a sexual union and not some other kind of union; it's a union in which the rights and responsibilities of men and women toward each other -- and toward the children of their union -- are publicly defined and supported, not merely left up to individuals to figure out privately.
Why do so many diverse societies arrive at this core marriage idea? There is something special about unions of husband and wife.
The answer is not hard to see. When a baby is born, a mother is bound to be somewhere close by. But if we want fathers to be there for children, and the mothers of their children, biology alone will not take us very far. We need a cultural mechanism to connect fathers to the mother-child bond. We also need an institution that communicates to the next generation -- in the throes of its own erotic and romantic dramas -- how seriously society takes the need to discipline those dramas so that children do not get hurt.