Over at Reason magazine, Damon Root writes there is no legitimate Constitutional reason to define marriage as an opposite sex union.

"Supporters of Prop. 8 claim that banning gay marriage advances a state interest in procreation. But if that’s true, why not ban infertile individuals from getting married as well? Or perhaps the government should require childbirth as a condition of the marriage license?"

Let me answer Damon's question: because doing so would almost surely decrease the likelihood that children are born to and raised by married couples.

Such a legal structure would not strengthen the relationship between marriage and procreation, it would weaken it.

More people attracted to the opposite sex would be refused marriage, and so would be more likely to create out of wedlock children as a result of alternative sexual relationships.

Here's another way to paraphrase Root's question: Why is it that the law has never barred infertile couples from marrying, but has barred impotent men from doing so?

Why is sex necessary for "civil marriage", but not fertility?

No, the state does not require you to prove sexual capacity (oooh, don't want to think about THAT test!) before issuing you a marriage license, but it does permit you to annul your civil marriage--i.e. have the state declare it null and void--if a man or woman was incapable of sexual intercourse when they married.

Wow.  Why?

Because here's the way that marriage protects children: it regulates sex.

The only way men and women attracted to the opposite sex can reasonably hope to protect their children by making sure they are united in one family with the man and woman who made them--is to FIRST enter a faithful, sexually exclusive, permanent sexual union.

The relationship between marriage and responsible procreation explains most of its key features under the civil law. Strip marriage of this public purpose, and it becomes, literally, unintelligible as a public, civil institution.

Damon Root admits this by saying, in his view, there really is no good reason why government is in the marriage business.  He's right, under his view,  there is not.