Crazy! I just did a quick search on it; apparently the guy who wrote it is really regretful about the book and has changed his perspective significantly since writing it. He even made a documentary about his revisitation of the book (I Survived ‘I Kissed Dating Goodbye’).
But you’re definitely right. By completely withholding information and keeping kids in the dark, they grow up to have a very distorted and unhealthy relationship with sex. I think you’ll enjoy ‘Pure’; it basically highlights some of the lasting effects that kind of teaching had some of those women.