So much of this resonates for me, but I do find it interesting that what you found easiest to find as far as other partners is very different than what I’ve experienced. In my life, finding another man for an MFM is by far the hardest thing. We even tried to hire someone but couldn’t find a male escort willing to do that. We did eventually find some men who enjoy that, but in my experience, there are a lot of guys who say they are into that, but who chicken out at the last minute.
In my experience (and obviously yours was different), men want to be in control in bed, and if there’s another guy there, they aren’t sure who is in charge. They are worried about the other man being territorial and possessive. The easiest thing to find (for us) is another couple. I guess it all depends where you are, and what the norms tend to be in that area.