So, just to be clear
You want a NTR GAME, that only shows the couple's day-to-day life, their dates, when they have sex, and the problems that exist in all relationships.
so that later, one day, the MC sees his partner's phone and discovers that she is being unfaithful, he confronted her and then black screen "Thanks for playing"??
Is that what you want?
No sex scenes, not thoughts of her, no sexual photos, nothing, 
Do you just want to see what happens 85% of the time when someone discovers they're being cheated on?
		
		
	 
What I would want is...
MC view only
Story development prior to the steal so that the characters are set in a situation where they show true emotion and care for each other. A good way to do this is to have the couple face some adversity before the story gets into the steal so that you have a good measure of their connection to each other.
Next the setup is the introduction of antagonist and it can go many ways as to why the FC begins to corrupt (blackmail, couple gets into some issues between each other and it leaves an opening, or the FC gets pushed hard by the antagonist in a way that begins to slowly corrupt her, the approach to this is can be numerous). 
Now the corruption. This is the part where the FC begins to stray. The MC starts to notice odd things (inconsistencies, gas lighting, changes in behavior, clothing, routine). This is where "close calls" and various other things can be introduced to the MC that are reasonable (MC is intelligent and perceptive, so the situations must be reasonable, not moronic "I see nothing" scenarios). You can implement false positives (ie MC thinks something is up, investigates and finds out it wasn't what he thought) as well as clever situations where the suspicion builds, but the MC is unable to catch the FC in a way that can solidify his suspicion, again... intelligent, reasoned, not idiotic. 
Corruption complete. FC has fallen, is stolen, but is still using the MC for various fall backs (money, amusment, etc...). The FC behavior gets worse, cocky and demeaning, even hateful (she has already fallen out of love, now the MC is just a cuck toy she and the antagonist joke about and play games of risk to humiliate him behind his back).
This leads to the MC eventually finding out in stages where his suspicion is realized and it can be a lead up of confirming situations (text messages, pictures confirming, video, or even obvious to the MC close calls by the FC). At this point, the MC can either learn about it before the FC knows the MC is aware, or it can be a apex surprise where the MC finds out first person (ie catches them straight in act). 
Then you have the resolution stage. Which will depend on how the MC finds out. If it is prior to the FC knowing, the MC could plan for a clever revenge using the ignorance of the FC/Antagonist as a means to trap/surprise/etc... them, or it could be an after process of the FC being caught and knowing the MC knows to which a revenge plan happens, or maybe it is just the MC kicks her out and moves on. There are lots of resolutions at this point that can occur depending on how the story unfolds.
Last is the after effect. What happened to the MC, the FC, and the antagonist. Was it a sad cuck ending (MC is dumped, cries and goes off into depression, etc...), does the MC find someone else, did the MC do revenge and what happened to the FC/antagonist, did they suffer any consequences, etc....
This would be a full cycle Netorare, all from the MCs view. Sex scenes would be limited to things like pictures, video and live reveals upon discovery (which could include humiliation, unwanted cucking, etc...) that leads to an ending of resolution and after effect.
Anyway, this is the sort of "NTR" story I like to read though it directly conflicts with those seeking the alternate view, or FC/Antagonist angle because their enjoyment is seeing the MC be used as a prop for the sex scenes.