Maybe they are trying to, unknowingly or not, protect their children from the burden of taking responsibility that may result in sex, like STD's or pregnancy. An immature child, in most cases, would handle such things poorly and thus the parent doesn't want them to even experiment with sex in fear of them growing up to be loose with it. It's just my thought off the top of my head though, it's quite possible im completely wrong. :)
The original question is thought-provoking, and Hexi has gotten off to a good start in considering it, I think. The entire package of anxieties about responsibility and consequences certainly seems to play a part in it. I wonder also if there is some level of envy involved.