I experienced something similar once when younger, on a wild, exploring whim. It was in a game of truth or dare in college in my early twenties. My gf at the time and I had been experimenting and exploring some, indirectly exploring videos we traded with a co-worker - pro videos, not ours. Anyway, this led is into some very basic exploring - of roleplay, positions, fantasy talk/roles, and eventually joining the other couple we'd exchanged videos with.
However, the experience that ties most directly in with this one involves a game of truth or dare we played. My gf had been turned on to inserting a finger in me by another friend who was in this game, which involved about 7 of us. The couple we exchanged some videos with was not present. But her friend, who was a tall, fiesty, fit redhead eventually was dared, or I was dared (it's all foggy now), but basically I ended up in a dare where she got to use a dildo (hand-guided not strap-on) on me. And this was in the game of truth or dare, with others, including my gf there, and watching some....Needless to say it was one of the most powerful, self-conscious, arousing, intense, submissive, awkward, and more experience in my life. Fuck, what I'd give to go back to times like that . . .