LaJan got a good point on that a responsible adult should know what he want to read or not. I want to emphasize it's READ. If you couldn't even think (for authors) or read (for readers), what freedom are we talking about? You can't punish someone for what he thinks, can you!
Underage sex, violence and people who enjoyed these activities are all over the places in the mainstream media: news, books, movies, etc.. So we shouldn't worry about to be arrested for what we think/read.
Well, to be honest, I do worry about it. If someday there were a law to forbid to think anything 'inappropriate', we would be all in trouble and this site would be closed for sure. Although this may not be happening, I can see some tendency toward that direction.
So when we are talking about our own freedom, please think if we give freedom to others.
Jinn