That is the EXACT situation a friend from high school found himself in. He was out of college, 22, in a bar, and picked someone up. Yep. 15. In Iowa, if you're 6 years or older than a 14 or 15 year old, it's rape. He was charged with a felony, and ended up pleading it down to something that just resulted in years worth of probation and probably on a few sex offender registries. Needless to say, his rising bank career was over, immediately.
According to his Linked in profile, he ended up being lucky, in that he put his life back together and went on to have a career, albeit out of the Home Town.
That is a toughie. The law is a law, and boinking a child should be a felony, but man, you drink a few beers in a place where everyone is 21+ one is bound to make assumptions that who you're taking home is also 21.