While you're young and expected to be high on hormones, this is a rather unusual occurrence. Merely being touched by someone from the opposite sex shouldn't elicit such a dramatic response. Do you periodically release yourself through masturbation? If not, you could consider doing so. If you already do, then there might be a deeper problem. Are you preoccupied with sexual thoughts for most of the day? Oftentimes, vising a good therapist for such problems can resolve it.  Don't worry - with the right help, you will overcome such reactions in the future.