Even polys have a problem with "cheating". At least all the ones I know (and have talked about this sort of stuff with).

They're open to sex outside of a monogamous couple, but instigating that without consent of all existing partners in the relationship is still problematic. They either have open pre agreed consent to sleep with anybody and everybody, or there's an agreed consent negotiation process on a case by case basis. Sleeping with other people without consent of the partner(s) is still cheating.

(I'll also note that "developing feelings" isn't a problem, at least not for me. You have feelings, and you have intellect, and feeling do not drive behaviour, intellect does. I have deep and lustful feelings for Kate Bush. My girlfriend knows this. She totally understands why and how I'm so head over heels in love with her (or, more realistically, in love with her public persona). Those feelings in no way diminish my feelings for my girlfriend, and even given the opportunity to act on those feelings I would not. In that vanishingly small chance I ever got to meet Kate socially, I'd do my very best to kindle a friendship with her. But I would not leave my girlfriend to "take a shot at her", or cheat on may girlfriend with her no matter how much opportunity I had to do so.)