Imagine you have a relationship where one of the rules each person is expected to follow is "Do not have sex with other people." You both agreed to it. Then you find out the other person broke it. Trust is gone. This would be different than someone saying "Hey, I know we agreed to this rule, but can we revisit that?" and having a grownup discussion about ENM alternatives, where someone has the opportunity to say that is a dealbreaker for them or declare boundaries that make this OK with them. Nonmonogamy is cool if everyone is aware and onboard. Fuck cheaters. If you're going to claim monogamy without actually being monogamous, don't enter into a relationship on those terms and save everyone the grief/waste of time/psychological damage.