I know you will want to know why it started, or how. I never liked riding the bus, but had no other way to get to work. Because the bus is almost always full by the time I get on, I have to stand all the way downtown. The trip takes about 15 minutes. The bus gets more and more crowded, until the driver won't stop to pick up anyone else! By that time we are really packed in together.
I am always horny; always have been. But not any hornier than other guys my age which is 21.
About a month after I started taking the bus, there was a middle aged woman standing in front of me. I had not noted her face, but the tight skirt on her butt looked really nice, her blouse pulled into her narrow waist. Her breasts seemed nice too, but I found myself staring at those beautiful round buttocks.
As the bus filled more and more, she was being pushed back toward me. I really couldn't back up any more, and despite my trying to avoid it (really, I think I did.. maybe...sort of?), her soft round warm butt pushed back against my crotch. Not that I wasn't ready, my cock had started filling up when I realized that she might get moved back against me. But WOW, when she actually pushed back against my erection, talk about an instant reaction.