He admitted to having sex with her and she was 17 at the time, which in California (but not in 40 other states) is a crime. It's no longer an accusation if it's proven/agreed upon to have happened. What was in question was whether she portrayed herself to be of legal age, which is an acceptable defense in California. Now on the face of it, it's absurd to think that a high schooler would bother going to a college party and then tell people he/she is 17, but you need to prove that it didn't happen and that takes time. Given the reaction, not only by the public but by several players on the Bills, the Bills couldn't afford to keep him on the team.