I think it is a matter of degree..
Exposing small children to sexual situations is considered negligence under the law (at least it was in the state where I took child-abuse classes) and will result having your children removed.
But there is a big difference between having porn movies on a computer your six year old can get to, vs Junior at the age of 16 looking at my pictures and rubbing one out.
Personally if I were a parent, I'd be more worried about Junior playing Grand Theft Auto in his room and fantasizing about killing people, than I would about him looking at my tits.

But for whatever reason unknown to me, fantasizing about killing people is a lot more socially acceptable than fantasizing about fucking them.