My first bi experience was with couple I met online and my first threesome. I was 21 at the time, they were around 40. I still considered myself completely straight at the time, and they never said beforehand that they wanted male/male play.
I was fucking his wife, her on top, suddenly I feel something wet on my asshole. It was the husband, he put his finger in my ass, and as I soon realized he was lubing me up. Next thing I know he lifted my legs up and his cock was pressing against my anus. I tried to get up, but the wife pushed me down and told me to relax, just as he entered me. First it hurt for a second, then it just felt weird. Not good, not bad, just really weird. At this point I really didn't know how to react, so I just kind of went with it. The husband began thrusting in and out slowly but steadily, and to my surprise it started to feel good after a little while. In the end I came while he was fucking me, and he came inside my ass.
Turns out they had planned on this since the beginning. Guess I should have been mad that they took advantage of me like that, and maybe I was a little, but I still kept seeing them, and after that him fucking me became a regular part of our sessions. I think this event was a big reason I discovered I was bi.