in my own experience, children touch themsaelves, as do we adults. If you make an issue to early..you could interfere with important self discovery behavors.
When i was 11 my mother would often be in the bathroom below my room listening. She would run up the stairs and beat me..if she heard me at all. Its an inocent thing when you are young...however, it changes your perception of people, sex and yourself...if your not understanding.
So long as you keep his behavior in check when in the public sector..and find out WHERE the "sex" thing originated, he should be fine. Remember, we are human beings and as such are naturally nosey. However, the social schemes we have been born into modify our thinking and behavior..not always for the best.
You dont say what he is doing if it is touching himself yes most children do this , liitle boys seem to wander around touching themselves oblivious of their surroundings, I think it depends if he is touching others or doing it obsessively you may want further answersthere are sometimes when it is a learned behavior, so always be aware of who with and where you child is going..Good Luck