Sir Brian, you must understand I wish not to trivialize such a heinous act such as rape. As I said, a more fitting punishment is to play leapfrog with unicorns. It would be a true injustice if both parties received anywhere near similar punishments.
A plea bargain for rape is nothing to be taken lightly. The law would much prefer a confession and 100% certainty that they have them and can punish them than risk a trial, where anything can happen. In a trial, even if the boys plead guilty, a jury could still find them innocent. Think of it less as a lighter punishment, and more as assurance on the law's part.
Lord Dane makes a good point, that future on the Sex Offender registry is nothing to be trivialized either. With their records sealed, it's probably worse, since they have nothing to disprove everyone's initial thoughts of "Child Molester!"