Interesting story. Unlike you, I was always polyamorous, even when I was in a monogamous relationship, it was merely accepting that those restrictions exist and I have to play by those rules if I wanted a relationship.
I've been in a poly relationship of one sort or another for over 10 years. The main thing that baffles me about yours is the no-talking-about-it rule. If I get my heart broken, or something bad happens with someone else, the first person I want to talk to about it is my SO. I have a real hard time understanding how I'd deal with that without being able to tell them.
This also opens you up to a lot more lying than is needed, simply my SO asking how was my day when I had sex with someone else, forces me to lie. I don't want to lie to my SO.
Oh, I was able to talk to him about having my heart broken, otherwise it would just be very confusing with me crying all the time during that haha. When other minor stuff happened, like if someone I was seeing was being a knob or anything romantically related to someone else I could just talk to my best friend instead. While it would have been great to talk to my partner about it I still felt it was worth it to have this rule and it worked well for us. Everyone is different and every relationship is unique, this rule would perhaps not work with other partners. And when he asked me how my day had been when I had been with someone else I simply said everything but left out the sex stuff, so it was not fully lying, just not telling the full story :)