That the behavior is unacceptable is hardly communicated, but what is communicated is that the behavior has limits. Violence is OK, but making Mary scream when mother is trying to chat on the phone is not OK - that's the message. The child then begins to learn how to express their violent tendencies where they won't get caught. Children in the playpen with each other, or even alone, cannot , however, get into sex play. Where the purported dangers of sex play - venereal disease and pregnancy, cannot possibly be present in the playpen, nevertheless the child is instantly told by the tone of the mother's voice and the intensity and quickness of her actions that such play is a serious infraction.
Adults are intensely uncomfortable when a child's curious probing finger goes into the diaper.
If such curious play has come to the mother's attention, the child is likely not to be left alone during play, and most certainly won't be left alone with other children if sex play has begun. The anxiety and resulting anger and fear that the mother expresses speak mountains to the toddler, who often develops such a parallel anxiety about sex that they are crippled for life in this arena.