Comment 19 by Scruddy Bleensaver :

When the state gets involved with tax benefits, etc, however. you're looking at social engineering. Clearly society benefits when children are raised in stable families. Just as clearly, society is not helped by incestuous unions between brothers and sisters, therefore those are not to be so encouraged.

Gay marriages fall in the middle. I can easily see why the benefits of the social contract (again, hospital visitations, etc) should be allowed. I can't see the benefit of same-sex marriages to society, though, nor do I see why they are (from the point of view of society) superior to polygamous unions.

Well plenty of children are raised perfectly well by couples who have two uteruses, and the evidence is that having no uterus at all is no detriment to the child. So gay marriages, at least the ones which are likely to produce offspring, don't "fall in the middle" of this classification at all. Whereas marriages of people beyond childrearing age, whether gay or straight, would.

The way to square this circle is to admit that the state has no business choosing between different configurations of genitalia, or (in my view) even the number. Where it does have a role is in promoting stable family units in which to raise children.

This doesn't require any particular institution. In its simplest form, it could be achieved by allowing tax allowances to be moved around a stable, child-rearing family unit. In a sane system, this would include the allowances of the children themselves, but that is a discussion for another day.