The unfortunate fact is that some adults use children for sexual gratification. In some cases it's one of the horrible legacy problems: they were sexually abused by someone, so they're now playing out the abuser role. Some people make excuses for it, but in the current culture in mainstream US there is simply no allowable excuse for it. No persons in a body over a certain age should be touching persons in a body under a certain age. Law is kinda strict on that and needs to be, however arbitrary the numbers are from place to place.
Our culture and society simply does not prepare children for the emotional impact of sexual contact. Period. There are other cultures that are otherwise, but violation of the rules doesn't make those children prepared, no matter what other children in another place are properly prepared. Our cultures, our language, our standards, all bleed into everything we do in this country/context, and this is simply not acceptable behavior.
When children have inappropriate sexual contact with older persons, an inner boundary is being breached. Regardless of the age of the body, there are still hormones and emotions that are engaged that are inappropriate for children and children are ill prepared to handle. A child is not ready for the commitments that sexuality implies and entails.
Because the emotional concept of physical sexual contact includes a commitment in this culture, to have sexual contact with a child still carries with it a factor of emotional depth and context: one of commitment and attachment to the child. However, the relationship is not that of TV or other real life couples the child witnesses; this one is one which is couched in secrecy. If most people who are in-love celebrate it with parties, ring exchanges, huge widely-attended weddings dressed in expensive clothes followed by all-night festivals (receptions), etc. why is it that a child who is being sexually abused must hide their feelings and closeness of their relationship with the person who is sexually abusing them? Is this person ashamed of their relationship? Ashamed of them? The child usually comes to the conclusion that there must be something very wrong with them, or some reason the other person is ashamed of the relationship, which leads to shame in the child.
Aside from the overwhelming damage that it can do to a child's emotions and self-esteem, sexual abuse can also carry a high physical price. Insertion of inappropriately sized objects into various orafaces can result in painful and/or permanent physical damage or even death (Truddi Chase, in When Rabbit Howls gives a very graphic description of what can happen to a very young baby forced to fellate a man). It can also carry a price that bleeds over into future relationships, causing a person to be immobilized by normal sexual contact, or inhibiting the forming of healthy relationships later in life.
At the same time, the damage that sexual abuse carries can often be healed.
Sexual abuse is thought to create some alters who are promiscuous and others that are highly sexually repressed.