Her story:
The cross-town bus was full and I had to stand in the aisle. I was going to visit my boyfriend for a day of sex, so I was too busy thinking about what we were going to do to find the crush annoying. But the driver was jerky and I couldn't reach a hanger, so I was being thrown around a bit. On one particularly violent lurch I grabbed the shoulder of the guy sitting by me, and then leant against him as the bus took a corner too fast. I felt him lean into me to steady me. Without thinking I stayed resting my hip against his shoulder: it was more stable that way.
I was conscious of the heat of his body through my thin cotton dress. I glanced down, but all I could see was a head of fair hair and a white shirt. The feel of his shoulder against my hip sent me off into an erotic daydream about my boyfriend: how he would grab me as soon as I came in the door, tear off my dress and take me on the carpet. I had deliberately left off my bra and panties in order to make this process quicker: they usually got ripped anyway.
Gradually I became aware of something stroking the back of my thigh, just above the knee. At first it blended with my reverie but then I realised it was my neighbour; I could tell by the movement of his shoulder that it was his hand that was slowly and gently working its way up my thigh. It was in fact very erotic, and I didn't protest. In fact my daydream grew ever more detailed and arousing.
The hand was now within an inch of my crotch. I shifted my legs apart to enable closer contact. He touched my lips and I imagined his surprise at finding (a) no panties and (b) wetness already slicking the soft folds of skin. His fingers continued their upward journey and pushed slowly but firmly into my wet slit. This was unbelievably exciting. I looked around the bus, but no-one seemed to be noticing. He must have had two or three fingers right in me, another finger touching my tingling rosebud, something else (his thumb?) gently massaging my rear entrance. Wow! I could take a lot of this! I only hoped he would make me cum before I had to get off the bus.