It’s only cheating if it’s not a part of the plan, whether it’s a relationship or anything else.

In go fish, lying about what you have in your hand is cheating. In a closed book exam, using your notes is. Nobody says that people want to play a bluffing game or take an open-book exam because they want to cheat. At most, people will say that someone wants those things because they will perform better under those circumstances, which, while not phrased well, is also true of poly people in monogamous relationships (and very much vice versa- especially for monogamous people who can only conceive of polyamory as cheating), no? It’s not a choice to be polyamorous, and people who are won’t be as fulfilled by a monogamous relationship.

That doesn’t mean they’ll cheat on a monogamous partner, because that is a choice (and frankly, I suspect poly people are less likely to cheat than monogamous people because of the emotional work they’ve already done to realize that they’re poly- I only know one poly person who is a serial cheater, and that’s because he refuses to do any of the required emotional labor to make enm work, so he does just continually get into “monogamous” relationships; whereas I know many monogamous people who are currently or were previously serial cheaters).

Additionally, just as looking at your opponent’s cards or your classmate’s answers are still cheating in bluffing games and open book tests, dating or romantically/sexually interacting with someone outside of what is agreed upon in a poly relationship is still cheating. Poly people don’t have an excuse to cheat at all, and in fact have a greater responsibility not to, given the larger number of people whose health depends on their transparency.