It seems to boil down to Larry Iannaccone’s model of religion:
Bearman and Brückner have also identified a peculiar dilemma: in some
schools, if too many teens pledge, the effort basically collapses.
Pledgers apparently gather strength from the sense that they are an
embattled minority; once their numbers exceed thirty per cent, and
proclaimed chastity becomes the norm, that special identity is lost.
With such a fragile formula, it’s hard to imagine how educators can
ever get it right: once the self-proclaimed virgin clique hits the
thirty-one-per-cent mark, suddenly it’s Sodom and Gomorrah.
Here is much more, on the general question of why the teenage children of evangelicals get pregnant so often. The first question is whether they do, adjusting for all the proper demographics. Second, I wonder if there isn’t also a combined lifecycle/genetic effect. Maybe if you’re rowdy when you’re young, you’re religious when you’re old, but the kids that pop out are on average rowdy too.