I am in a similar position to you MonoVCPHG. Was best friends first, deeply attracted. Their marriage opened, and he and I became lovers. It's been going on for many years now. He has become a tantra educator, has had an other lover during the time I left him due to his new condition that he have more lovers.
When in full love, I just can't get how someone can share same with someone else. It feels like the universe is laughing at me. If he were to go for another lover now, I would leave him in a flash.
I too am in full authentic compersion regarding their marriage. His wife has opened herself to two other lovers which he gets into bed with and engages though he says he does not have intercourse with them.
Any reference to his sexual engagement with others through tantra practices, his wife's lovers, the lover he recently had, have caused my mind to imagine the acts and it has become self torture. I have learned to manage this mind over the last year having returned to him and this new paradigm, but become overwhelmed with all of it when too much is going on.
Basically, I have asked him to keep his sexual engagements to himself. But it's hard to go to our special places knowing he brought his last lover to those same places. It is hard for him to have to keep his personal sexual life out of our conversations. And when I reach my limit of coping, express my feelings that usually contain unresolved resentment, he reacts strongly.
Now he asks me to take a good look at myself and figure out whether I belong with him. I have actually been doing that all along. He calls me his 'slipper baby'.
I wish I could come upon my own monogamous partner, or maybe a poly one who'd be open to the continuation of this relationship. The imbalance here is crazy making. His life is full of sexual activity, including me in a major way. My heart has slowly retracted even though he states it to be unlikely he'll take on another lover, unless in a call for healing.
I am at a loss right now. I am prepared to leave him because my heart has detached quite a bit over the last months. Perhaps this is the time I can tell my sub-conscious and conscious mind that his sex with others is not really an issue anymore.
I've always thought the only way I can handle all this is if my heart pulls away. Now that that's happened, maybe I can be ok to hang around, and actually be poly myself. At least until a mono shows up in my life.
I have been a cup half empty life evaluator. And that's why I think I've brought this situation into my life. I am getting closer to seeing the cup half full and filling up, having deeply worked with myself, emotions and spiritual base. But not there yet.
Any responses will be welcome.