Sorry. You are absolutely right. Let's go back to my statement:
The idea that we are somehow beyond the need for the state to involve itself in the regulation of procreation is laughable. Marriage is the institution around which this regulation centers.
The reason for this regulation of procreation (even before the state entered the picture) is to maintain stable family structures. I must have said that on other threads and neglected to make it clear here.
Look, I know you are trying to argue that because the government doesn't perform fertility tests that that can't be the purpose of marriage. NutCrackerBoy and I simply reject that logic. That's just not how it works.
Again, it gets back to the promotion versus prohibition concept. We are not prohibiting certain people from joining together in a private union or even having children, we are promoting the best possible family structure for doing so by providing legal advantages for this structure. The fact that people take advantage of that legal benefit without using it for its express purpose is immaterial, as long as it accomplishes its overall goal.
Furthermore, a reliable conclusive test for infertility has yet to be developed. Yes there are certain physical conditions where it becomes clear that infertility is a near certainty, but for couples in general there's no way to gauge fertility with certainty. So it's just not a practical test. Again, it doesn't matter, because the government's goal is achieved even if people get married without having children, because the majority of married couples do at least try to.
Finally, even if a couple is infertile, they can choose to adopt. And frankly the best family structure for adoptive children is the same as for bioligical children: a stable heterosexual marriage. Hence there is still an interest in promoting the marriage among infertile couples.
But if insist want to go down this road, where the government cannot promote am institution unless only its purpose is fulfulled, that's fine. I am more than willing to entertain a new legal structure in which the full protections of marriage are reserved only for heterosexual couples with children. In other words, you would still get married in a personal ceremony, but full legal status would kick in only upon the birth or adoption of a child.