Jacob Levy asks an interesting question which I'd like to involve myself in only tangentially.
Suppose that a state legislature forbade recognition of, or even (on the model of the polygamy statutes) criminalized, marriages between persons at least one of whom was known to be infertile. Suppose that it did so for the stated purpose of affirming the societal commitment to marriage's cerntral function as the primary site of childrearing.(All spelling/grammar mistakes are his.)
Would such a statute be constitutional (under the federal or most state constitutions), according to the jurisprudential theories of those most strongly opposed to the Massachusetts case?
Rather than address the legal issue (or the gay-marriage issue), I'd like to disagree with anyone who believes that the primary purpose of marriage is to have children. That's a commonly-held conservative/Christian position (apparently), but I think it's absurd. For one thing, the first mention of marriage in the Bible says nothing about children whatsoever.
Genesis 2:18-25I find it difficult for any Christian to argue that marriage is all about having kids.
18 The LORD God said, "It is not good for the man to be alone. I will make a helper suitable for him."
19 Now the LORD God had formed out of the ground all the beasts of the field and all the birds of the air. He brought them to the man to see what he would name them; and whatever the man called each living creature, that was its name. 20 So the man gave names to all the livestock, the birds of the air and all the beasts of the field.
But for Adam no suitable helper was found. 21 So the LORD God caused the man to fall into a deep sleep; and while he was sleeping, he took one of the man's ribs and closed up the place with flesh. 22 Then the LORD God made a woman from the rib he had taken out of the man, and he brought her to the man.
23 The man said,
"This is now bone of my bones
and flesh of my flesh;
she shall be called 'woman,'
for she was taken out of man."
24 For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh.
25 The man and his wife were both naked, and they felt no shame.
What's a family? A family is a husband and wife. You don't need children to be a "family" -- a husband and wife are a family all on their own. Children are great, and get added into the family later, but the primary and most important familial relationship is that between the husband and wife. In all cases their first loyalty should be to each other, not to their parents, not to their children, not to their siblings.
Husbands and wives should always be in public agreement on every issue, all the time. That doesn't mean that there won't ever be internal disagreement and discussion, but a unified public front should always be presented to all outsiders, with no exceptions. "Outsiders" include children and other family members, as well as friends, and everyone else. Each partner should subordinate all their other earthly relationships to their marriage.
If the purpose of marriage isn't children, what is it? Well, the passage above makes it pretty clear: the purpose of marriage is provide helpers to assist each other in serving God.