But I’m afraid the theory is at bottom mostly a theory of why Bryan thinks his wife should have more kids, rather than a theory of what will make people in general better off.
In all honesty, I don't mind this kind of speculation; I've wondered about it myself. But I don't see that this has much to do with whether I'm right or wrong. (Will only mentions one part of a much bigger argument).
Now we move onto Will's substantive point:
[I]t seems mighty, um, gendered to me, failing to take the opportunity costs of childbearing for women very seriously at all. A sixteen year-old girl gets pregnant, and the perspective Bryan assumes is that of a grandparent? Weird.
...But for most sixteen year-olds, the cost of having a kid is simply immense, possibly destroying any serious ambitions before they develop. So what’s the problem with a woman who decides to devote her life to meaningful life-constituting projects that do not involve stretch marks and minivans? ... Reading Bryan on kids, you sometimes gets the sense that he thinks the primary function of women is to serve as incubators of grandfatherly delight.
Actually, I'm assuming the perspective of a parent, just like virtually all the people denouncing Jamie-Lynn.
But in any case, if I'm reading Will correctly, he's saying that there is a gender conflict on the issue of family size: Men want more kids than woman because women get pregnant and do most of the child-care. The problem with this story is that, empirically, desired family size for men and women is practically identical. In the GSS, the average response to the "ideal family size" question is 2.57 kids for men versus 2.56 for women. Indeed, if there is a gender conflict on this issue that the data doesn't capture, it's between women who want kids, and men who don't. (See I Want a Baby; He Doesn't).
Now admittedly, Will could rephrase his question in a gender-neutral way: "So what’s the problem with a person who decides to devote his/her life to meaningful life-constituting projects that do not involve [insert gender-neutral unaesthetic stuff about kids here]?" If so, my short answer would be:
1. It's not hard for a person to do both, especially if he/she is reasonably affluent.
2. A person who does both will almost certainly be glad that he/she did (whereas many successful childless people (especially women!) regret their choice.
3. The person you create will almost certainly be really glad to exist.
4. If you think you own any debt of gratitude to your parents, giving them grandchildren is the best way to repay it.
P.S. What's all this stuff about grandfathers? Last time I checked, women desperate to become grandmas were a lot easier to find than men desperate to become grandpas.