Because a sexual assult is not the same thing legaly as rape. For example if a person grabs a womans breast that is a sexual assault but not a rape. If you show your self to a child that is a lewd with a person under 14 not a rape. A common tactic in law is to plea bargin. I would not be suprised if on further examination they decide to try him for a lesser count.
People take things like this to heart, but people should only be imprisioned by what can be proved. If they cant prove he can have an erection, nor prove he used other means to penetrate the girl then they should let him go. Most cases are won and lost in interogations. Whom ever cracks first looses. In this day and age of "want to see my lawyer". It is hard to see who is hiding the whole truth until you get to trial.