Really depends on who or what that "other version" actually is. If you have a MeSeeks box, Shadow Clones or an outright hivemind those are disposable toys or extensions of "your" own body. In a stable time loop future-you already knows how past-you felt about it and that is the standard to judge it by. Genetic clones, alternate universes and various other timey-wimey shenanigans basically produce completely separate characters who share many traits with "you" and count more like twins.
Personally most of it comes down to the writing though. Naruto can transform a clone into Sasuke and !Sasuke can keep prattling on and on about how he's so much better than Naruto at everything including banging both their wives, that could quickly become textbook NTR in terms of presentation even though factually it's all self-inflicted acting. On the other hand if you, your girlfriend and bad-future-action-hero-you decide to have a heartfelt farewell threesome as the rift closes the "not technically cheating" excuse works and you could even frame it as a romantic extra-special date.
Personally most of it comes down to the writing though. Naruto can transform a clone into Sasuke and !Sasuke can keep prattling on and on about how he's so much better than Naruto at everything including banging both their wives, that could quickly become textbook NTR in terms of presentation even though factually it's all self-inflicted acting. On the other hand if you, your girlfriend and bad-future-action-hero-you decide to have a heartfelt farewell threesome as the rift closes the "not technically cheating" excuse works and you could even frame it as a romantic extra-special date.