Despite your grotesque example, the girl without a leg would be better off alive and without a leg, than with a leg and dead. And Margaux apparently was better off with Peter in her life than without him, despite the problems that came about. (As with the others, I am saddened to hear of Margaux's passing.)
But you ignore another important point:
With legality and public acknowledgement, we could improve behavior.
First, note that we could still have a law with a higher standard for sexual relations with children than with adults. Before the U.S. pressured their government to change the law, the Netherlands simply refused to prosecute statutory rape at age 12 or higher without a complaint by the child or their parents. No one else had a say. We could also have a law against penetration by a penis for children beneath a certain age, which most of us would be fine with. Alternately, we could keep the laws as they are but just get rid of the draconian punishments - make the punishment fit the crime. Criminologists have found that what deters crime is not the severity of the punishment, but the certainty of the punishment. And right now, the severity of the punishment works against the certainty of punishment. After all, consider Margaux's situation: she loved Peter even in the midst of a troublesome relationship, and that relationship was good for her overall. What are the chances that a girl in such a situation will seek help with her relationship, when any help she seeks will certainly result in the end of that relationship, and severe punishment for someone she loves?
On the other hand, if Margaux had known that a first offense would simply get Peter a severe talking to, and other offenses would get punishments weighted in accordance with the offense, she could seek help without fearing for her loved one's life. If it were completely legal, or had only some moderate restrictions, she would have even more opportunity to seek help. Instead of being the one who helped four year old Joan, she could be playing the role of Joan, getting someone older to help her.
Of course, in such an open environment we could also be teaching men like Peter how better to communicate with little girls, and we could establish channels whereby girls who wanted a relationship with an older man on their terms could get help communicating those terms to their men. Openness allows far more options than a blanket ban.