(The following was posted to John & Belle Have A Blog's entry on Just Not So Stories)
I don't think that either the Just So or Just Not So stories work, and in general the explanatory power of Just So stories is weak unless you're sure that there is a genetic predisposition to be explained. For instance, if male mammals or hominids in general preferred younger mates, it would be much less of a stretch to suppose that a similar male human preference has a genetic component.
The problem with your Just Not So is that it assumes that just because you have an explanation of why behavior A would be arguably more beneficial than behavior B, selection pressure couldn't have produced B, but evolution is more complicated than that. There are any number of reasons that sub-optimal behaviors could have been selected for, even assuming that your analysis of the merits demonstrates greater optimality (it might not, e.g. better ability to run while carrying offspring--proxied by a more youthful, fit appearance-- might trump ability to bear them, particularly since sperm is cheap). For instance, preference B could be the result of an "arms race" (much as female peacocks's preference for longer tails); starting from a preference for obviously childbearing women, a slight pressure to select the younger appearing could snowball until it was the appearance of youth itself that was preferred, despite the preference being sub-optimal or even maladaptive.
The same sort of objection can be raised against the original Just So story: in the absence of evidence for the genetic predisposition being explained, it's overreaching to attempt to account for behavior this way. Even if it were true that the indicators were proxies for fertility, that's not enough to establish that fertility (and not, say, ability to raise offspring to reproductive age) is obviously the winning genetic strategy. Humans, after all, are K-reproductive strategists, not R-reproductive (lots of care into few offspring, not little care into many).
That's all a pretty long-winded way of saying that while the Just So story to explain male attraction to young women is underwhelming, it's not so easy to demonstrate that it can't be in part a genetic predisposition just by reasoning about what's likely to succeed as an evolutionary strategy.