The hottest thing that happened to me...
This happened about five years ago at a safe-sex/masturbation party. I had been to several before, but had always gone alone. They were pretty hot. Usually about ten to fifteen men and five or six women, some of them in couples. Everyone was very respectful and well-behaved. (Sadly, the group does not seem to exist anymore.)
When I heard about the party coming up one time, I sent out emails to a bunch of friends, a social mailing list, asking if anyone wanted to go with me to the next such party. One of my ex-girlfriends, who I'm still friends with, said she would go, and talked one of her female friends into coming along. They're both in their late thirties. Then my ex had to drop out due to being out of town or something, but her friend, who I had known for quite a while, and we were friendly but not close buddies and I had never dated her, said she would still go.
I picked her up, and was nervous driving there, since I had never run into anyone I knew at those parties, or gone with friends. I wondered if she would be shy or inhibited (although in real life she's not at all inhibited!) with me there. She had seen me naked before (hot tubs, etc.) but not with an erection and I had seen her naked but not in a sexual situation. So I was not sure if she was going to play, or if (like some of the women) she would just keep her clothes on and watch.
When we got there I took my clothes off, and she stripped to a fishnet see-through top and short-shorts. Guys started following her around, but she joined up with another girl and they went to an alcove to be more alone. (I'd guessed she was bi but didn't know for sure.) I didn't know if she wanted me to hang out with her, or what, so I just wandered off, and joined another little group of three men and one woman. I was jacking with them for a while, which was cool, but I was really interested in what my friend was doing.
Then she and the other woman emerged from the alcove along with a guy who had arrived with the other woman, and the two women sat down on a couch. Me and the other guy sat on the floor next to them. By then my friend seemed pretty aroused, and it was clear she didn't mind that I was there. I was as hard as a rock and stroking.
She took off her top and played with her nipples, and was only wearing a pair of panties. The four of us jacked and jilled, and a bunch of people, maybe six or seven, were watching. It was incredibly hot to see this woman I knew as a casual friend rubbing herself through her panties. The other woman said, 'you're really wet, why not take off your panties', and my friend did, and she put her fingers on her clit and looked over at me and raised her eyebrows as if to say 'how am I doing?' and that totally set me off and I came harder than I ever have, and the other guy came too. My friend came about a minute or two later and collapsed onto the other woman.
I've been to other parties and clubs like that but for some reason that was the hottest scene ever. I really hope to do it again.