Now, we have no way of knowing exactly what happened. The girl says one thing, the prosecutor believes another. He probably believes something happened because either 1) the girl at one point said there was (she may have been lying at the time), 2) there is some physical evidence (think Monica Lewinsky's blue dress), 3) there is a witness (who may be lying), or 4) there may be a hearsay witness (who may be lying).
Or 5) Something DID happen, but the sexual activity was absolutely mutual and in no way coerced, the two are bf and gf, and she is simply refusing to rat him out. This is the most likely possibility, and your refusal to acknowledge this, coupled with your smoke-and-mirrors trick of deflecting attention from this possibility, speaks volumes about YOU. I've known enough teen girls in my life to know that a good many of them would love to date a 20-year-old and would certainly consent to sex with him.