I wish I knew the age of your son. Since you talk about other mothers having caught your son, I’m guessing he’s old enough for parents to feel concerned. It’s possible that he’s expressing normal curiosity. In this case you need, with the help of children’s books, to inform him about the differences between boys and girls. You also need to let him know there are consequences when he disobeys you and doesn’t respect the privacy of others. It’s also possible that another child has “played with” or abused your son so that he has become preoccupied with sex. Children who have been abused often will get involved in sex play with other children. Therefore, if this behavior persists, I suggest you consult a child psychologist.