Since this is from my primary boyfriend...
I feel like i should say something. I started out dating him as a secondary, and was actually cheating on my ex who put me through a lot and was (I'm about certain) cheating on me. It was awful and I couldn't handle the drama. So when I met my primary and started to see him sexually I made it clear I didn't want to compete with any other women, I just needed a friend and no strings sex. He agreed readily and as time went we got closer and began dating in earnest, although still in secret. My Ex and I were engaged but had a very stressful and unhealthy relationship, and i eventually left him. Meanwhile, my boyfriend (OP) made it clear he felt no jealousy over me having sex with other men and I had some casual sex (some of which he loosely participated in with me) and that was it. When I finally managed to end my relationship with my then fiance, I offered to be monogamous. I knew I was not emotionally ready to deal with other people. I said i was ok with him seeing other men in a strictly sexual way (he's bi and I can't meet those needs) and that I would be interested in the future in possibly having some more experiences with a third person in our bed. We bot joined a dating site, and I met my latest boyfriend, who I initially was just sleeping with. Before and after every meeting I asked my primary if he was ok with me seeing someone else, and how far the relationship could go. I asked before agreeing to date him, and constantly said I was willing to be monogamous because I knew it was unfair of me to expect him not to see other women (something I ask of both my partners). He insisted it was fine, and then thought I was going to suddenly open u and become "truly poly" despite my saying that I wasn't ready and his constantly bringing it up (when our own single relationship needs work) was very stressful for me.