It's happened quite a few times to me, and my reaction depends on who's doing it and where. The most memorable time was one morning on the Piccadilly Line in London. A gorgeous fit guy, who looked Middle-Eastern, and probably in his late 20s got on and stood in front of me. He was holding a leather file in one hand and as the train got busier, the back of his hand brushed my crotch. I moved out of the way, assuming it was just an accidental brush. Then he swapped his folder to the other hand and cupped my balls and started to rub me up and down. I had already moved as far back as I could and, as I was against the end wall of the carriage, I had nowhere else to go and didn't want to cause a scene, but he carried on stroking and I got hard. Then he moved again so that my cock was positioned between his cheeks and he began to rub his backside up and down, flexing his buns. I have never been so turned on in my life, but we arrived at my stop and I had to get off and go to work. Not my most productive day in the office, but it certainly provided some wank fodder for a while after that. I told my other half about it, and every now and then, when we're on a train, he'll rub up against me.