When I was a senior in college my roommate had a girlfriend who slept over on quite a regular basis. She was very pretty, although slightly heavy. Her long curly black hair brushed her tan skin in a way that always got me so horny. I knew they regularly had sex, but they only did this when I was out of the room, as far as I knew.
One night, I went to bed around 12. My roommate went to go pick up his girlfriend and he brought her back to our place. I was still awake when they came back, but I didn't feel like talking to them, so I pretended to be asleep. After about 10 or 15 minutes they got into bed and I still hadn't really fallen asleep. They must have really thought I was asleep because a few minutes later I heard Marissa moaning and I realized that my roommate was fingering her less than five feet from me. She was very vocal and it really turned me on. I instantly got a stiffy thinking of her heavy breasts and pink nipples and my roommate's fingers slipping in and out of her slippery pussy.
To this day I don't know how I managed to do it without waking them up, but I took off my shorts and started stroking myself under the covers. I was so turned on and I just figured, hey, if they're two kids having fun with each other, why can't I have fun by myself. I felt like even if they found out what I was doing they wouldn't mind and might even let me join in (although they probably wouldn't really have-but that's what I was thinking at the time). So I stroked my hard cock really slowly and lightly so I wouldn't make a lot of noise. I finally came, the sperm gushing by the gallon, it seemed.
That was probably one of the most intense orgasms I've had from masturbating. I sometimes wish that I let them know what I had done. Who knows what that might have turned into. I would have loved to masturbate with them. That was the first time I ever realized how sex is just supposed to be really fun, no matter what the circumstances or who is involved.
I still think about that night and masturbate to very strong orgasms. In fact I think I'll do that right now!