This may not be helpful to you. I hope in some fashion, it is.
I was in a long-term married relationship and went through something similar, although, maybe not quite as intense. The beginning of my tale was about 25 years ago. My wife made the same suggestions to me about being with other people. When she mentioned this I was very surprised. It came out of nowhere. I was very reluctant, and at the same time, very interested.
So, off we went down this path with me being with other women, swinger clubs, meeting couples, etc. We kept this up on and off for a few years. To make a long and hurtful story short, it ultimately ended in disaster. The sordid details are not that important anyway, except to say I received a lot of pressure in weird ways from my wife in the beginning and many times across the years. I have never been able to piece it all together as to why so many things were ok then not-ok; this was ok, but that was not. Ultimately, it was about more fundamental issues than having relations with other people. The sex stuff may have been more of a symptom than the issue.
Reading your posts has helped me rethink those early interactions and potentially explain many lingering confusions and hurt feelings. In hindsight, we simply did not communicate well. She didn't communicate to explain what was happening with her (like already being with others). I did not communicate either and ensure we were really talking apples to apples. I was complicit and mostly having fun. I have to own that. It is water under my bridge now, but I would tell my old self (meaning my previous self. As Dylan said, "I'm younger than that now.") to have the right conversations, no matter what, no matter how difficult.