What is your opinion on someone who is poly having ‘casual’ sex with people who are not one of their partners despite one (or more) partners being uncomfortable with it? Would you consider this cheating or simply them being inconsiderate/disrespectful?
It doesn’t really matter what I, a person not involved in that relationship, would consider it. If to you, something is a dealbreaker, then break the deal.
If your partner consistently does something that makes you feel disrespected, is that part of a larger pattern of not respecting your wishes, or is this just one boundary they are pushing back against? If it’s part of a larger pattern vs. one thing they just cannot/will not do for you, that’s important.
Have these feelings been made clear and discussed? If someone says “if you do this thing, I will consider it cheating, and act accordingly,” and their partner says “Well, I refuse to stop doing this thing,” then both parties need to seriously discuss whether to end the relationship or re-negotiate the terms. People have the right to do what they want; their partners have the right to respond in ways that work for them.
It’s only ever up to an individual what they consider intolerable or a dealbreaker. Other people’s opinions don’t have the power to validate or invalidate someone’s feelings, choices, or boundaries.