He argues that "the reason for marriage is neither solely to produce children, nor to seek romantic fulfillment, nor merely to contract with the government for rights or benefits. I propose another model, arguing that it explains the institution of marriage much better than the common reasons given for it in the same-sex marriage debate."
Here's an excerpt (but please read all of the article!):
Marriage is not, as Young and Waring imply, merely about choosing a steady sexual partner. On the contrary, it is a reciprocal agreement with another individual (and often with God), to look after the total well-being of that person and of any children that might come into your mutual care.
This total well-being encompasses all aspects of life, including not just the sexual, but also the spiritual, social, economic, psychological, and physiological best interests of the partner. Ideally, it lasts from the time the marriage is solemnized until the death of one of the partners.
It cheapens the covenant to say that marriage is just about sex, or just about rights, or just about children. Marriage is about all of this — and more. Marriage is a complete, all-encompassing, nurturing relationship. It’s about care for the whole person, so much so that no one else in all the world is quite as important.
(Hat tip: E at Paradoxy)