I'm with Dawncry on this one. If it's part of the backstory before the player had any control and it's just a memory of something that already happened then that's fine. If it's something the player very clearly did something to deserve then it's fine. If the whole game is focused on that theme then it's fine if it's unavoidable but that should be pretty clear from the beginning. She's going to cheat on you no matter what. Some people are into that, but it should not come as a surprise.
If the game implies that the wife/GF is not out to cheat on you no matter what then NTR should not be unavoidable. It should also not be something you can accidently stumble into. One wrong answer to some choice where the consequences are not very clear should not trigger it. Some trivial "infraction" by MC should not trigger it. Unless the game is premised on MC's SO being a complete slut it should not feel like she's just dying to fuck every guy she meets unless MC spends every waking moment trying to prevent it. It's fine if MC's SO is a total slut as long as that's not really a surprise.
If the game implies that the wife/GF is not out to cheat on you no matter what then NTR should not be unavoidable. It should also not be something you can accidently stumble into. One wrong answer to some choice where the consequences are not very clear should not trigger it. Some trivial "infraction" by MC should not trigger it. Unless the game is premised on MC's SO being a complete slut it should not feel like she's just dying to fuck every guy she meets unless MC spends every waking moment trying to prevent it. It's fine if MC's SO is a total slut as long as that's not really a surprise.