Vice has an interview with the self-appointed poster man for frotteurism — the act of rubbing one's genitalia against a non-consenting person. But it sounds like you'll learn more reading Dickflash.
"Is this one of us?" reads a message-board headline. Attached is that video of the woman confronting her subway flasher. But the board commenters aren't talking about her. Welcome to Dickflash, a twisted forum where flashers trade tips and tricks.