Before we were married my wife and I were friends with another couple I'd known from college. I'd returned as a mature student, they were both eight years younger than us and I'd fancied Fran for as long as I'd known her. I knew both she and my wife were curious about being with another woman but neither had tried anything.
We held a party one weekend and everyone left except Fran and her boyfriend who lived a long way from us so we invited them to stay. We offered them the couch, the spare bedroom or our bed and although nervous we all decided to sleep in the same bed, the two girls in the middle.
We were all naked and the girls started kissing with myself and Fran's boyfriend stroking their bodies. The curtains were open and the moonlight was enough for us to raise ourselves up and watch them explore each others mouths and breasts. They masturbated each other as they took it in turns to kiss and lick each others nipples and the whole thing was very slow and sensual. I felt Fran's fingers wrap around my cock and guide me into my wife and I moved slowly in and out of her as Fran stroked her clit. My wife had a long orgasm that made her toes curl as Fran and her boyfriend sucked on her nipples.
We stopped and talked for a while and Fran told us she couldn't bring herself to give Tom oral sex, she just didn't like it, much to his disappointment. My wife offered to suck him and looked to me for my reaction and I said that I'd love to watch. Fran asked my wife how she felt about the two of us fucking while she sucked Tom and she smiled and kissed me and told me she'd love it.
Fran and Tom lay side by side and kissed and carressed each other as I gently fucked Fran and my wife sucked her boyfriend. It was incredibly erotic and didn't take long for my wife to make Tom cum who thought she would pull away but she held her mouth over him and I thought his head was going to explode. Fran asked my wife not to swallow, sat up to kiss her and then came as she tasted her boyfriend in my wife's mouth. I pulled out and wanked myself over Fran's stomach but both girls began kissing me, traces of Tom's cum on their lips still which just sent me over the top and I came all over her amazing body.
After that Fran still coudn't bring herself to give head but loved the taste of cum.
They moved in with us a week or so later and stayed for more than a month before we drifted apart and each moved away.

That was the only same sex, group sex encounter we've ever had and we'd love to repeat it but both recognise there was something unique and special that would be hard to recreate.