Say that someone intentionally taps you on the shoulder to get your attention, or intentionally pats you on the back to compliment you, or even touches your arm in conversation or hugs you when parting. You might be slightly put off, at least under some circumstances, but the law would (and, I think, should) consider this to be well within the boundaries of permissible behavior. Not all unwanted touchings are batteries.
Say, on the other hand, that someone intentionally touches your genitals, or intentionally caresses your breasts (if you're a woman). In many circumstances, this would be considered a crime. Why the difference? I think that here too there is a connection with sexual arousal — either the possibility that you might be involuntarily sexually aroused, or the likelihood that the other person is deriving some sort of sexual arousal from touching you. [UPDATE: For more on the latter point, which on reflection I think I probably underemphasized, see here. Also, to make clear what I thought would obviously be clear when we were talking about nonconsensual and illegal touching -- and what I expressly said in All Related Posts (on one page) | Some Related Posts: