here’s my take
before doing something, consider this: would you be happy to tell your partner what you’re doing? if not have you specifically said it’s ok to do but you won’t be discussing it?
by “happy to” i don’t mean that have to or should do, i mean how would you feel about it
if you’re not comfortable talking about it with your partner, maybe you shouldn’t be doing it… and if you shouldn’t be doing it it might be cheating
open relationships are of course a very complicated point relating to this, thus my “have you said you won’t be discussing it” caveat