To be fair, that's absolutely a thing. Generally speaking, polyamorous or open relationships tend to have very strictly defined guidelines or rules set up in advance specifically to avoid hard feelings and people breaking up when they realize they're happier with their side piece than they are with their current main partner, and in some ways breaking those rules is actually seen as almost being worse than normal cheating. For example, having a long-term steady relationship with one specific person and keeping that a secret from your main partner is absolutely considered cheating in a lot of those circles. As would be continuing to see a specific person if your main partner basically says "I'm not comfortable with you seeing that person." Basically, it shows a profound lack of trust and respect for your partner, which is sort of the entire point of a relationship at all.
That also doesn't even get into more complicated issues, like whether or not both partners in a relationship actually want to be in an open relationship in the first place, or are just humoring the partner who really wants it because they're desperately (if likely futilely) trying to preserve the relationship in some way out of fear of abandonment. Or if one partner pushes for an open relationship but then gets bitter because the other person seems to be getting way more side than they are.
Open relationships tend to be a mess in general because of that sort of knotty morality.
i never thought id say this, but america needs more jesus