Cheating is the act of breaking a rule and trying to get away with it. If you’re not trying to get away with it, then you’re not cheating. I can think of two reasons why someone would want to break the rules on purpose. The first reason is that the consequences for breaking the rules is desirable. An example would be if someone got stuck in a competition that they don’t want to participate in, they can break the rules and get disqualified to get out of it. The other reason for blatantly breaking rules is to show disrespect towards the entity that is sanctioning the rules. Just to be thorough, I guess there’s also a third reason where the person is breaking the rules due to ignorant dumbassery.
Cheating becomes ambiguous when the rules are not clearly defined. That is the case for couples. Now I’m not suggesting that new couples should clearly define what constitutes as cheating for them. That doesn’t work because once you define a set of rules, that means everything else is fair game. It would be silly to expect horny people getting together to design airtight rules. Even thousand page laws have loopholes.
Without the exact set of rules, sometimes it’s hard to know if you’re cheating or not. Just because you have sex with another person, it doesn’t necessarily mean you’re cheating. There are happy wife swappers and swingers out there. It’s within their rules, so it’s not cheating. The only other way to determine whether a person is cheating is to see whether they’re being sneaky about it. If they’re doing something that they don’t want their partner to know, then it’s probably cheating. It might not be, but it probably is. It all depends whether you believe a person should be innocent until proven guilty, or guilty until proven innocent.