[M] My time with a couple

My story starts years ago, I was in University (undergrad), 19-20 years old at the time. I had just gotten out a short and intense relationship which was just non-stop sex, between classes, after the bars, any time.
I took a few weeks to myself to just think about what I wanted. Eventually, I did as most late teens, early 20’s do and downloaded some “dating apps”. Time went by, and I ended up with a match that immediately followed-up with a message.
Female, 30’s, great pics. She was fairly fit, hiker, outdoorsy type. The message starts of with the person saying that it is actually the fiancé, or her husband. And that he is looking for someone to have sex with his wife in front of him.
At first I was a little shocked, this was the first I’d ever encountered anything like this. I was curious. Things took off from there and eventually we settled on a night.