Totally understand how you feel as l was in this situation.
Met a guy - polyamorous. In long term relationship with a married polyamorous woman (E) whose husband also poly.
Was completely fine with it. I knew there was no "danger" of him wanting more as he loved his partner. I read the Ethical Slut book and really liked what l read - communication, boundaries, agreements, openness.
I knew that l would feel more secure if l knew if he planned on sleeping with anyone other then E and so we had that agreement.
Until he did.
He told me after the event as he "didn't know if would happen". Apparently they had been flirting and been suggestive to each other for weeks and he went to her house for dinner. ...
It was at that point l realised he didn't follow the Ethical Slut "rules".... and l stopped reading the book.
More also happened and it completely messed with my head.
If he is willing to reassure you and you're able to come to agreements ... like the book recommends... then good.
Unfortunately, we couldn't as l asked him for 3 things - the above, having the next date of meeting in the diary...even if it was 2 weeks away... and for him to give me foreplay or to finish me off after he'd cum.
He didn't abide by the first two... and the last one he refused as he "didn't have the full Spectrum of feelings" for me - only E.... and therefore it felt awkward for him.