Tyler Cowen muses:

[R]aising children is the main thing that goes on in a marriage, yet few of us choose life partners on that basis. The film [Knocked Up] suggests that a random allocation might be better than selecting a partner on the grounds of smarts, common interests, attractiveness, how good he or she makes us feel, and so on.

He goes on to offer a few considerations as to why this might be true or false, but misses what I would have thought was the big one: A married household will provide a good environment for kids insofar as the parents are able to keep each other happy and sane, to model an affectionate relationship for the kids, and so on. When, in my teens, my parents separated, I remember realizing that there must have been ongoing tension or arguments for some time, and being so grateful that they had done their best not to expose us to that. I can still remember pretty vividly the one real fight I did happen across, and while it was hardly traumatic (some raised voices, no nasty namecalling) it’s not a fond memory either. So maybe there’s an invisible hand here, too: Being good for (and to) each other is one of the best ways of being good for the kids. Next to that, I suspect moderately divergent parenting philosophies just aren’t all that important.