"You don't know what it - okay," says Bella, putting a hand over her eyes and engaging Explaining Humans Mode in spite of Nona being a human already. "By default, if two humans are being all snuggly and lovey with each other, they don't also have sex with anyone else. If they do it anyway, that's called "cheating". People make different arrangements, sometimes, and obeying the rules of a different arrangement isn't cheating, but apparently you didn't make one with Etty, and the only reason this hasn't bitten you before is because you live in a castle in the middle of nowhere. This is not a small deal; people end decades-long marriages and kill each other over cheating. It is a form of betrayal. The fact that you apparently aren't familiar with the word is probably a mitigating factor."