This happened last year on halloween night.
A very attractive friend of mine, (who I had always wanted to be sexual with but never really got the opportunity,) was helping me out with a job early in the day. It was fairly boring work, so we spent most of our time chatting and making jokes. Eventually those jokes became of a sexual nature, and the rest of the day was filled with nonstop flirting.
After many, many hours, we wrapped up our work and decided to chill out back at my place and see if we could find any scary movies to watch on television. So I unfolded the pullout couch in my living room and we opened a bottle of homemade wine. There was nothing scary on, so we ended up watching a really stupid softcore porn about vampires. Now, after a glass of wine and two hours of fairly hot sex scenes, one cannot be blamed for the horomone driven activity within one's pants.
I was feeling fairly bold, and I simply said to my friend, 'If I were by myself, I would probably be masturbating by now.' She simply smiled at me with a very devlish look. I could tell the concept had been in her mind for a while anyway. After a few more minutes, I simply reached into my pants and slowly pulled it out. I was already quite hard, and I just started stroking it right there beside her.
The idea of having such a beautiful girl watching me do this was extremely arousing, and quickly I became solid as a rock. I decided to make it a bit of a show for her, and made it a little more theatrical than usual. All the while I had been a bit too nervous to look at her, but out of the corner of my eye I could see her eyes were fixed on the action. I tried my hand at teasing her a bit... just as a test. I stopped what I was doing and gave her a fake little apology, claiming that I was a little drunk. However she quickly responded, saying that she didn't mind at all, and she had never really had a chance to watch something like this before. Grinning, I went back to it. I was very leisurely about it for a while, but it didn't take me long to really get into it. I guess all the fun and games were a bit too much for her, as I glanced over just in time to watch her hand slide down the front of her pants. Now I had achieved my goal for the evening >: )
She kept her eyes on my swifly pumping hand as she rubbed herself. After a little while her other hand crawled up her shirt, (which did NOT house a bra), and the sight of her fingers moving around her nipples almost killed me. By now my attention was focused on watching her masturbate. The image was so hot and so personal that I had to stop touching myself for a little while! Eventually she flipped her shirt up over her breasts just enough so that her nipples were exposed to view. She would pinch them a little and then rub them in circles, all the while busy with the work of her other hand. I could sense that she was building toward climax, I so desperately wanted to take over for her, but I didn't want to do anything to potentially end this perfect evening. So I just picked up where I had left off and kept pace with her for the next minute or so... then it all became a blur. I just remember her throwing her head back and grabbing the side of the couch while she did her best to silence a scream of pleasure. The whole image was intense and passionate and VERY HOT.
After that I just remember having a spectacular end to my own excercise and nearly passing out. It was an amazing experience, and the fist of many.