Suppose a previously-monogamous couple opens up their relationship, but as the years go by, neither person feels inclined to start any new relationships (nor have sex with any third party). If this continues to be the case until both people pass away, then do we call that monogamy or polyamory? (I'm not sure what the right answer is.)
I think that's an irrelevant mental exercise focused on how best to categorize relationships which require no categorizing.
In an *ideal* open relationship, cheating wouldn't occur, for the reasons you cite.... I don't think their differences are as clear-cut as that.
"Cheating" is a function of someone acting against an agreed upon rule, whether the rule was explicit or assumed. If these boards have a story to tell there are AMPLE opportunities to cheat in a poly relationship, possibly even more opportunities than in a monogamous relationship.
Granted, that's not a form of relating I will be associated with, because my goal is "less rules" not "more partners". As always, any perspective about polyamory is either confirmed or denied by *which* polyamory you are talking about.