Oh, I think that's the exact thing - the EC books already have sex. That, and a lot of th fandom are people who are 40 and don't have a fascination with sex because they are well past that phase in life where sex is "OMGSEX."

In HP however, the books are PG-13, and even the kissing scene wasn't really shown. Readers who are going through that "what is sex like" stage are going to write it or read it in fanficcc since they can't read it in canon.