Is cheating only a thing monogamous people experience or does it happen in polyamory too? Does it mostly depend on if any boundaries are set or like the dynamic of the relationship?
Cheating can happen in any relationship, if you define cheating as “violating the established terms of the relationship.” You are correct that the more “boundaries” that are set, the higher the likelihood of “cheating” becomes. If someone decides that their partner is allowed to have sex with other people and that’s not cheating, but if they “catch feelings” or use specific words or have sex without getting permission first or do a specific sex act, then that counts as cheating, then they’re certainly at risk of being cheated on in that context.
Polyamory changes the definition and boundaries of “cheating,” for sure. Simply having a sexual-romantic relationship with someone else doesn’t necessarily meet the threshold for cheating, which can really simplify things. But every polyamorous individual might feel threatened by different things, so it’s important to be clear and honest about what you would consider “cheating.” One thing polyamory forces us to do is actually name and claim our individual needs - monogamy lets you rest on a lot of cultural assumptions about what is and isn’t ‘allowed’ within the bounds of a relationship. (It is possible to do this intentional work in a monogamous relationship, but more optional.)
Also, for a lot of polyamorous people, “cheating” is a useless concept, but “dealbreakers” and “violations” are more meaningful. There are certainly behaviors my partners could engage in that would lead me to immediately end the relationship and to consider them ‘unfaithful’ to me as a person and to our relationship: violence, lying, manipulation, cruelty, etc. All the connotations of ‘cheating’ - a violation of trust, a betrayal of the fundamental connection that forms the relationship, a requirement for immediate changes to the relationship - are definitely present, but it has nothing to do with a specific sexual-romantic behavior.