but I don't believe that happened in the books.

In that interview on his death scene Martin explicitly points out that Joffrey is still just a 13-year old boy in the books. This is in reference to his bullying but I think it's safe to assume that 13-year old boys are just as naive (and nervous) about sex. I mean, it's not like there's sex ed or anything back then, and Joffrey *was* thrust into the kingship rather prematurely. Assuming a "normal" trajectory to the throne, then he would have had plenty of time to go through puberty without the exposure or responsibility of ruling the kingdom.