Please, please, don't make the cheating (netorare) path a "punishment path" for being a wimp. The rarest gem is a game where cheating comes as a choice of the NPC, regardless of the player's actions building up to it. In a majority of games, NPCs are props without any depth to them, they merely react to the player's choices. Be nice to them and they love you, be nasty and they get back at you. NTR is the "reward" of the player picking options that only an asshole would, hurting their LI and making their relationship sour. Avoid it by being "manly" or caring towards the LI. It makes the player feel as if the whole story is hanging on strings they can pull at will.
A lot of players do enjoy ego-raising stories about alpha main characters that get to fuck every girl in the game. A lot of others enjoy playing from the perspective of a submissive pushover. I can see why authors create alternative paths that cater for both... What I almost never see is a game that walks the line between extremes. In most "NTR optional" games, the player gets some choice, and they're stuck with being either alpha or beta for the rest of the game. And cheating is, per usual, reserved for betas. But why?
For my part I'd like to see an NPC have their own agenda, regardless of player decisions. Your LI cheats on you, that doesn't make you a beta. Deal with it. "Be a man", scold your girl for being a cheat, make her cry and tear her hair in regret, put your rival in their place... Like, no. Vanilla players won't like it, NTR interested people won't either. As an alternative, the choice that leads to being cheated on could be somewhere else. Maybe you took a long walk through the park, your LI was alone at the time and had a chance meeting with some stranger that stirred their interest, even if things were good between you. You get an option: don't take a long walk that day, and you'll never see NTR in the game. Take the walk: you get cheated on, not because you did bad things that upset your lover, but because life is complex... Sometimes, sucks. But, then, you saw them, you have to act, no?
Here, again, a lot of games adhere to stereotypes. You hate it, but don't have the balls to do anything about it? You're done, you're on a hard NTR route, on your way to a bad ending and getting your LI stolen from you. Some people like this... Or you like it, you "discover yourself" in a previously unknown cuckold fetish, and you become a submissive cuck to your LI and their bull... Some people like this, too. But what about actually trying to fix things, like adults do? You hate it, you love your LI, you respect their individuality - they're not your puppet - and you're self-confident enough to not go about trying to "win them over", defeating your rival, whatever. You're smart and frank, you create the right mood to talk about it, tell them you hated seeing them with someone else, but ultimately they're grown ups, who own their choices and their actions. What options lie ahead? To open your relationship? To try swinging as a healing process, to perhaps dispel some taboo and defeat some demons?
Hundreds of possibilities, with the trick being to not adhere to stereotypes and try to pull some surprises from the characters, not from the player, "the puppet master"...
Thanks for your game, good luck with it!