I think I failed to make my point completely... Didn't say the game shouldn't WARN the player that a given choice would lead to NTR. What I meant was that the choice itself didn't have to be of the "crime-punishment" type. The "walk in the park" example: the player gets to choose whether to go directly home, or take a walk by the park. They're not doing anything that elicits cheating from their LI, they merely create an opportunity for hazard to play in. During that extra alone time, the LI meets a stranger and surrenders to an impulse. I don't think this is far-fetched, happens all the time... But why did I suggest it? Because making NTR a punishment path takes all the fun out of it. I don't like playing asshole characters who make dumb choices and get cucked for their own stupidity. But if the MC were to make a totally innocent choice that allowed for NTR, I might take it and see where it would lead to. Important point: the opportunity to opt out would be there, if the author placed a warning on the choice. That would allow players to avoid NTR, or embrace it, without splitting the story between smart MC and dumb MC. Player input would be there.
Exploring an NTR route that weren't a punishment for the MC's fault of character probably would allow for much more interesting interactions, and put a greater focus on the LI's own feelings, conflicts and fears. Most games lack this, all the NPCs are doing there is reacting to player actions, sometimes making them behave so differently on different paths that it's hard to believe they're the same character.
Also tried to say that you don't need to label a character as a cheater, in order to have them cheat, or prepare the player for that possibility. Just give them a clear opt-out, but let characters be more dynamic. I do see your point in the sense that your LI cheating just because they had the opportunity to do so, sounds cheap (and doesn't cast a good light on them). But that's hardly an issue, if the story can later explore the motive. People repress their fantasies all the time... Maybe that could have been the case? Suddenly, the LI's repressed fantasy burst out during an accidental encounter, and they decided to live it, even if it meant trouble ahead with the MC. Don't people do that all the time? You don't even have to label them "a cheat" AFTER they cheated, as if their whole character had to be reduced to that one, possibly wrong, choice. We all fail to meet our standards sometimes, and our standards themselves may shift. Labelling people (real or ficticious) is self-righteous and hypocritical. But, if a character in a story is presented in a shallow and simplistic manner, labelling is almost inevitable.