friction does it for me too. any time a chick is dancing with me and really is rubbing her ass hard on my dick i will usually get a boner. it hangs down when soft and when i get hard it will harden following my leg because my boxer's leg will hold it in place. so when she is grinding me she can feel my hard dick running down my leg an no one else is the wiser.
good times.
embarassing story would be this one, it popped in my head as soon as i read the thread name. I was in the 9th grade gym class. it was a co-ed class and we were doing indoor putt putt (dont ask). i was in a group of five and one the girls i knew decently and the rest i didnt know at all and ditto for her, so we hung out together while playing. well this is chick is really fine, she had the nicest face, so cute i could pinch her cheeks all day, but that wasnt the cheeks that i was interested in. she had one of nicest asses ever! it was beautiful. she was wearing short shorts, u know the kind, made out of cotton and real tight that bunches up forming the perfect outline of their ass. whew i am all and bothered now.
anyways, while i was sitting down in the only chair at the putt putt green the chick was putting and i was staring at her ass so hard and i was getting a semi. she had just finished and she was walking over. at that point i had a full hard on. she then walks over and says "wanna share?" i had no idea what she meant but i said "sure". then just turns around and sits right on my lap! my dick poked her right in the ass!! she jumped up and said "what the hell!" i then said back, "dang, almost a hole in one." (best line ever). she kinda had a smirk on her face but she masked it with a fake frown and she just walked away.
Btw, i fucked 6 months later