It's called frotteurism: rubbing against a nonconsensual person to achieve sexual arousal, typically in a public place such as a crowded train.  And the District is a city of frotteurs.  That's my only explanation for the amount of unwanted touching that keeps happening to me.  I don't even ride the metro.

First scenario.  It's crowded, you're trying to get through, but my back is to you.  I would expect that you would tap me on the shoulder.  Upon realizing you were there, I would move and let you pass grope free.  Instead, you place your entire hand on my shoulder, and then midpass, you even add a second hand to my back.  Gross.  Now I feel I've gotten a sensual massage *and* hepatitis. 

Second scenario.  It's crowded, you're trying to get through, I see you coming and move out of the way.  At this point there is no need to place your hand on my shoulder as you walk by.  It's not like you're trying to get my attention - I've already seen you and moved out of the way.  But you do it anyway.  This is creepy behavior - don't be surprised if I remove your hand in a less than cordial manner.

Third scenario.  I'm sitting at the bar, you come up from behind and place your arm around me like I'm your best friend and start muttering in drunkspeak.  Oh, hell no.  Dude, that's a good way to get stabbed in the neck with a pen that says "Toledo Lounge" on it.  In no way will I abide a bodily embrace by a stranger, especially one who looked and smelled like he woke up this morning in the gutter.

There is one form of touching that I don't really mind - you know, how girls sometimes grab your wrist or touch your arm when they're telling you a story.  I always thought this was a subtle flirtation and found it flattering.  Then I was talking to a lesbian the other day and she did the same thing to me.  So maybe it's not a flirty thing, it's just a girl thing?  That blew my mind.  All these years, I've been living an illusion.  Bah.

Anyway, the point is: stop touching me.  All of you.  You fucking sickos.