Ok, so I will try to say this with grace. And I won't dig into your comments about cheating vs not cheating.
Ok, I lied. I will say one thing about cheating. Cheating in polyamory is almost identical to cheating in monogamy. It's a lie. It's deceit. It's disrespect. It's stepping outside of agreements you have and breaking them. Mono marriage is an agreement.
The nuance of what's cheating, flirting, talking, etc., is still a discussion and a boundary point. I am a huge flirt. So is my wife. Even when we dated and played at mono, we always flirted incessantly with others. Flirting wasn't my version of cheating, but could be yours. Those are your boundaries to express.
So his "definition of cheating" is already likely bullshit. If he cheated (you don't say) then that trust has been violated
Ok, off my high horse. Onto the grace part. I have been poly 14 years (I always have to think about this) and non-mono for 25 or 26. I lose track. I have seen a lot of mono-poly attempts, and of all the ones I have seen or been around, it only worked once.
It required the mono person (there are no requirements for the poly person beyond honesty, ethics and transparency) to be almost completely independent and autonomous. It effectively requires them to be single while married and be extremely happy in that state. In the end, a mono person in a poly relationship is lonely. And if you don't have attachment issues normally, they will pop up. The worst of attachment styles will appear, and honestly, it's because your nervous system is telling you the truth and you don't like it, at your core.
Your nervous system is telling you your partner isn't loyal, may not stay with you, enjoys someone more than you, is sleeping with someone you don't know, etc., etc. All those demons cause your attachment style to go insane, and I might add, likely not get your needs met, since people will throw around the words "couples privilege" like toilet paper.
So, how this singular mono person survived:
- Was very very aware of his needs, and expressed them
- Was extremely conscious about his boundaries and enforced them with respect
- Enjoyed life with or without their partner, within the constraints of the needs and boundaries
- Never had those trusts violated
You don't say if he cheated, but if he did, then that last one is already crossed off, making the rest very difficult to overcome.
Best of luck. Truly.