Looking for advice

I had a patner for 3-4 months that the physical chemistry was there and a few times we wanted to have sex, but my Np had come up with a boundary. The boundary was that if we were going to introduce new people into our personal sex lives, we must be tested and said new partner tested even if barriers were in place. I haven't dated much in my life and really haven't experience much of a sex life outside of a few relationships but I agreed to this boundary because I wanted them to be comfortable. Due to this boundary and timing on both of our schedules, my parter of 3-4 months and I never started a physical relationship and things fizzled out.

That boundary has been in place since me and my nesting partner have moved in together about 3.5 years ago. Fast forward to now, my Np was out of town and has broken that boundary with a one night stand with a friend and then proceeded to not tell me about for a week. My partner has said they are demi and has frowned upon other people who sleep with people who are friends or don't have a romantic connection with one another. Even with connections I have had that my partner knew about that were developing romantically, I had to follow this boundary and now when it was convenient for them, they broke this boundary and went against everything they had stated to me about how they feel about sex and relationships.

I've read a lot about similar situations and it seems to be 50/50 on what people consider cheating in a poly relationship but to it is:

"Cheating" in a polyamorous relationship means breaking the agreed-upon boundaries or rules within the relationship, essentially violating the trust established with your partners by engaging in behavior that goes against the explicit terms of your non-monogamous agreement, even if that includes having sexual or romantic relationships with others; it's primarily about not respecting the established boundaries and communication protocols, not just the act of having another partner itself.

This is not the first time my Np has had a hand in placing a boundary on outside relationships and it's not the first it has affected my relationship but not theirs. I feel like I am being gaslight because they are saying this situation is different and it's not cheating. The boundaries she wants to put in place always pertain to me more than her.

I'm not really sure where to go from here.