This sort of thing happens to me a lot, but under different circumstances. Whenever I am dancing with some little hottie, and she grinds her ass into my crotch, I get a reaction. Sometimes they back off, and sometimes its enough to make them not want to dance with me anymore. Then there are the times when they grind into it more, and then I know its on. More often than not, I just get a look, like, "I dont know what you are thinking buddy." I just want to say, "Hey, its not a hardon. Relax." That never goes over well though.