It's rather more serious a matter than some folk seem to think, to be honest.
Many sex offenders have a tendency to start off with relatively minor offences such as flashing, indecent exposure, groping strangers and so on, only to advance to more serious and possibly violent offences as they grow in confidence and the more minor offences begin to lose their fascination. The offender can become bored and look for something more serious instead.
What starts off as relatively minor criminal acts, and wanking over a stranger in public without their consent is a pretty unpleasant act, IMHO, can (though not necessarily will) lead to the offender graduating to much bigger (and worse) things later on in their career.