This story happened when I was 19.
I was hanging out one afternoon in the living room at my friends house watching TV. On the floor, laying prone and watching TV was his sister, approximately a year younger. I always thought she was kind of cute, and definitely thought she had a great ass. We were always teasing and flirting with each other.
Anyway, we were watching TV. My friend was watching too, but he was kind of oblivious to what was about to happen. His sister (I'll call her Kim) was lying in front of me and intensely watching TV. She was wearing a T-shirt and a tight pair of jeans... and that gorgeous ass of hers was just beckoning.
My shoes were off, and I just couldn't resist. I slowly stuck my left foot's big toe sort of under her ass and pointed it at the goal line. It definitely got her attention, but she didn't resist. Kim just parted her legs (those were nice too, btw) and let me plow in. Afer a few minutes Kim murmured in a hushed tone 'wait a second...' and slightly lifted her ass and parted her legs a little more so that I had greater access. I began wiggling my toe up and down and drove in slightly harder. Mean time, I can't believe her older brother wasn't picking up on this... or maybe he was ignoring it?
I had zilch experience with the opposite sex at this age, even though I was already 18... but I could tell she was really enjoying this. Her breathing was becoming halted and heavier, while she more or less hid her face away from me (and brother). I had a feeling that she was coming, and coming fast. Finally, she gave a little squeak as she shuddered. I had enough common sense to know that she'd had an orgasm. I still couldn't believe my friend didn't notice this!
I tried going in for orgasm two. However, she turned to me and exclaimed 'Smith!' (not my real name). Ah well... I guess that was enough for now and that the fun was over with, but it was a pretty neat experience. Unfortunately, the situation never rose again for another session. And of course, she didn't get the opportunity to reciprocate. Maybe if I was a little older and therefore bolder? Ah well...
I can't tell you how many times I've jerked off thinking about that time.