Touching someone on the shoulder is not sexual assault. Let's try to keep things in perspective, please--if simply touching a member of a particular group warrants jail time, that group is an overclass.
Shaking hands = fine.
Tapping shoulder or top outside of arm = less fine, but acceptable in a narrow range of situations.
Placing cupped hand on shoulder of a stranger, with squeezing or stroking, with "friendly banter" = really not okay.
Nope, still not worth jail time. Move away from the person or grab their hand and remove it from your shoulder. I'm sorry, but you do not get to have people arrested for giving you an unwanted shoulder rub. If they persist, then yeah, they're assaulting (and harassing) you. Again, let's have some perspective.
How about, as described in the original post, licking someone's shoulder tattoo?