Religion

Back in Bronze Age and Iron Age times, marriage happened shortly after puberty. Lifespans were too short to wait to reproduce. It really wasn't until the Industrial Age began that people waited for several years after the onset of puberty to marry. That was a problem, really, since their bodies wanted to get down and get it on, while social pressure tried to block those urges, often using religion as the tool. It never did work, really.

A good number of adolescents simply could not block out their biological imperatives with theological barriers. The drive to reproduce is probably the most powerful biological drive we have. It's essential that we, as mammals, make babies. So, theology, which is purely an intellectual and social phenomenon, simply cannot compete with a couple of kids whose hormones are telling them to get it on together.

And that's that, really. Religion cannot trump biology. It's simply not powerful enough to do that in most cases. Even the preacher's daughter is likely to get together with the nice boy down the street and do what mammals evolved to do - make more mammals.