Cheating on his wife may be jumping to conclusions. Maybe they have an open r'ship (he has the OP after all) and doesn't want to know any details if they don't really impact her day to day.
Jbean, I have recently had a similar experience to yours, leading to me breaking up with my married bf (altho 2 mos after the breakup, we are now communicating again).
I felt like a hypocrite too, when, after a few months of being together, and him only seeing me, he and my gf started wanting each other. We went ahead with that, and after some glitches, they settled into a FWB thing. Well and good. But then, after we moved near him, he started flirting with and dating others from ok cupid, and having sex with a couple FB friends, and people he met through a hobby group.
So, in the course of the 2nd year of our r'ship, he had sex of one kind or another with: me, his wife, my gf, 2 single women, a married MF couple, and then one more single woman. 8 people!
I finally broke it off with him 3 months into his (stormy, jealousy ridden) r'ship with the married couple.
I had 2 other men plus my gf when Ginger and I first met, and first dates with 2-3 men after I met Ginger (and broke it off with the 2 previous men). My heart wasn't really into it, because Ginger and my gf really met my r'ship needs. But Ginger wanted more, and I didn't realize that in the first year of our r'ship.
I found it very very very hard to adjust to him having a new lover every 2 months for over a year. I had no interest or right to veto, so I did the only thing I could, broke it off with him...
A month after we broke up, his couple broke up with him. The woman he had sex with after our breakup but before his 2nd breakup doesn't seem to want to get together with him again, so they just chat online.
It all became so much drama! I was going insane. But now that I've had a break from him, and recently started having some online talks with him to see where our heads are at now, I realize he does not WANT a revolving door of other lovers, just one other. He and his wife are not that intimate. They even live and sleep in different houses on their land. My gf doesn't come through with the "benefits" all that often.
So, my recent feelings are, if I do want to get back together with my bf, it will be as a platonic friend or a FWB. I felt too bonded with him from all the great sex we used to have. I tend to be "all in" in a r'ship or even friendships. I couldn't develop compersion for him and his dating. I think partly it's because he has Asperger's syndrome and doesn't understand me instinctively the way my gf does. Also, I have not approved of many of his lovers. Especially the married couple, because they were poly noobs and the h was extremely jealous and insisted he be there participating for all sex between his wife and Ginger. And Ginger went for that.
Also, Ginger had many health problems in the past year and a half, which led to frequent erectile dysfunction, which added to the stress and my confusion.
So, conclusions: I have complete compersion for my gf's other r'ships, but she is very discriminating and has only had 3 bfs in the 5 1/2 years we have been together. Ginger will continue to date others, but he isn't interested in juggling more than one other partner (besides me, his wife and my gf). But of course, when one is dating, one needs to kiss a lot of frogs before we find someone who really lives up to the promise.
It's a work in progress, very difficult and confusing and stressful. I have learned so much from this board to even be able to talk to Ginger about our needs, desires, emotions, plans. It's complicated! I wish you luck and strength.