I was standing on a crowded subway behind a hot girl, near the back of the car where no one was paying attention to us. I started rubbing on her.
At first, I grabbed a bar with both hands and put the front of my torso against her back. I stayed like this for a few minutes, working up a hard, while she gave no indication that she knew I was there.
Eventually, I put a hand on her hip. She said in a quiet, shaky voice "Please stop that." She didn't make any movement. I pretended not to hear her and stayed put.
A few minutes after that, the train came to a stop, and people started shuffling off. While people were moving around, I put my other arm around her and drew her into my lap. She flinched and let out a little whimper.
10 seconds after that, a scruffy old man grabbed me by the shoulder and told me "let her go and get the **** off the train at the next stop, or I'll tell the police about you." I was tempted to ignore him or hit him, but I didn't want to risk legal trouble, so I glowered at him, moved to the door, got off at the next stop, and took the next train home.
I enjoyed the experience, but never bothered to repeat it, since it seemed to me like I was inches away from something really bad that day.