MOOP!

Cities are full of people who are mentally disturbed in some way or another. It's upsetting when one of them chooses to include you in his skewed worldview, but it's definitely not your fault. Try to remember that you're good, we're good, the city is good, most of the world is good — he's the one who's broken. Everybody who sees him is on your side.

For future reference, the MBTA is cracking down on harassment on public transportation, and they have cameras on the buses. You would have been well within your rights to get up and tell the driver, "Hey, that guy who sat next to me is rubbing one off through his pants." He would've been thrown off the bus, if not arrested.