I'm poly; I don't have a personal definition of cheating in that context. My partner is a grown-up and can do what she wants; I can barely see how that's even my business, let alone my problem.

But if you pretend to follow a given set of rules, while actually secretly breaking them - that's pretty much the definition of cheating in any context.

If your partner would be not-OK with some activity, but you want to do it, then you either do it anyway and face the conflict head on, or you don't do it at all.

Selling nudes specifically - some would care a lot, some would be fine with it, and the only way to know is to ask. But sneaking around because you assume it'd be a dealbreaker if they found out - no bueno. No bueno at all.