Thanks to John Piper, here's a little more insight on the whole issue of teen sexuality and chastity among evangelicals. It turns out that the book I mentioned (without having read it) distinguishes between teens who take their faith very seriously and those who claim to be evangelical Christians but admit that they don't adhere to key Christian doctrines. The former (16% of American teenagers) are for the most part living morally pure lives while the latter group are not doing so well in the same area.
The devil is always in the details ---especially with statistics. We still have quite a job to do in encouraging nominal Christians to discover the joy and the obligation of living a holy life, but the picture is not as bleak as I thought it was at first.
See also this follow-up post by Caine at The Wandering Heretic.
