The punishment definitely did not fit the crime. What he did was embarrassing and degrading, so do something embarrassing and degrading to him -- like force him to do community service wearing a t-shirt that reads "I can't keep my hands to myself." Force him to watch Doris Day movies (I think there are fanny slaps there) while undergoing electroshock aversion therapy. Make him go to sensitivity training, anger management counseling, something -- but don't deprive him of his liberties (and his family of his provision) for one slap on the butt. That seems extreme.
If he doesn't learn his lesson and commits another drive-by fanny slap, by all means, hang him by his thumbs.
"Oh, please. The guy was a jerk, but calling this sexual assault seems a bit ridiculous. Should he be on the registered sex offenders list in addition to four years in prison for being a jerk? "
its not in prison, it's house arrest. If it's the first time he's ever done it, I dont know that he should be put on the sex offender registry, but if he does it again (or this is not the first time) then there is obviously a problem with him and he should be put on the list.