I was the second M in a MFM, the other MF were a married couple, High School sweethearts and married for 15 years plus. I was about 15 years younger.
We were at least twice a month thing for about 9 months. After the F started slipping me notes how she want to be with me alone, it was fun, but after that I had to cut it off. For the marriage and kids. It might of not came to of anything but mentally for me a was a mind melt. Because 1 of our ground rules was as long as everyone was in the same room.
Sadly it crossed the line But I would do it again in a heartbeat. It was very educational (due to my age) and we all had a blast.
I wouldn’t allow anybody to make you do something you are not ready or comfortable in going.
I would have some serious conversations with your partner to figure out what you want out of it but everybody needs to be on all the same page and have an open conversation about ground rules, both do’s and don’ts.
I wish the best to you and is enjoyable for all.