Don't get me wrong, I think it is utterly crazy that ANYONE was charged in this case. It is a victimless crime. But reading between the lines there's a certain puritanical undertone to all of it.
In most other Western countries this would not be illegal in this way. But the US is extremely religious and has quite an unhealthy attitude to teenage sex (which ironically results in one of the highest teenage pregnancy rates in the Western world also).
I genuinely wonder how the people who preside over cases like this (e.g. cops, prosecutor, even the judge) sleep at night. They're actually doing more harm to everyone (including society) than the supposed "crime."