What she doesn't know, won't hurt her!?!
A few years ago my ex-girlfriend (we were still friends) and a friend of hers (who I knew reasonably well) booked the weekend in a retreat away from the city. However, neither had a car and they asked if I would be kind enough to drive them down (only about 1.5 hours away) and in return they'd buy me dinner and I could stay the night.
My ex and I had, in the past, discussed such topics as threesomes, etc.-in particular with this friend of hers-but together hadn't done anything about it. She had broached the subject with her in the past in jest, but didn't get a very enthusiastic response.
In any case, I wasn't really expecting anything to happen so I didn't have my hopes up. But as the night wore on, I started to wonder if I could instigate something. However, after a discrete chat with my ex, she made it quite clear that there was no possibility of anything happening between the three of us. Apparently her friend had an inkling that I might be hoping for something, and she wanted my ex to know that, whilst flattered, she just wasn't interested.
Eventually we all drifted off on the makeshift 'bed' we had constructed on the floor from sleeping bags, camping matresses and cushions. My ex's friend ended up between my ex and me.
Now my ex's friend was quite an eyeful-blonde hair, beautiful face and DD tits! She lay asleep only inches from me-the ambient light only just sufficient for me to make out her nipples jutting through the oversized T-shirt she'd worn to bed. It was just too much for me, and I-very slowly and very quietly, pulled my cock out of my pants and started to stroke it.
The more I stroked the hornier I got, until I dared to reach over with my foot and prod my ex. After a few pokes I saw her peer over her friend's body. At first she had no idea what I wanted, but then she suddenly saw my hand wrapped around my cock. Her mouth was agape and she propped herself up on one elbow to watch.
This was enough to really get me going and it wasn't long before I could feel myself getting close to cumming. At this stage all I wanted was her friend to wake up! Alas, she didn't, but seconds later I had pushed my cock back into my pants, not wanting to make a mess, and came all over them.
The next morning as I was leaving I spoke to my ex alone for a few minutes. She still couldn't believe what I'd done. I asked her what her friend's reaction would be if she told her what I had done-she said she didn't think it would be an issue and had already planned to tell her what I did later that night!
That night I got an SMS from her friend. She wasn't offended or anything and even teased me about it a few times since. However, she still insisted that she wasn't interested in even just watching me.
It was pretty exciting at the time though!