I find myself inherently suspicious of US legal attempts to crack down on porn because 'cracking down on obscene content' is the justification that the CCP uses in China to censor many things that are clearly not porn.
I'd like to write some sarcastic reason for this, but I can't even think of one. I have a feeling that "one nation under God" has something to do with it, though.
The book is written from the perspective of a pedofile lusting after a young girl. Stanley Kubrik's film of the movie in 1962 won an academy award. There was an immense ammount of controversy surround the movie and the book, and they still are. Art or obscenity? We'll be having this discussion for years to come.
Neither movie contains anything explicit beyond kissing, as I recall. But there's no doubt what's going on.