As far as I can see, this isn't actually NTR. It is about sharing/swinging/netorase/cuckqueen.
The game is basically all about the consensual adventurous lifestyle, so I have a hard time picturing how the NTR would happen with this set up. Even if you don't pick choices as the male MC that encourages her to bang others, he basically is already somewhat accepting of an open relationship.
If you didn't really want to support a true NTR route, that is absolutely fine. But if you did want to implement that route, I'd say you need to be more deliberate in setting up this route as you move forward.
NTR is when the male MC doesn't want his girl to sleep around but she does so anyways. It's best if he doesn't know about her infidelity until much farther into the game. This is kind of opposed to the wholesome adventurous couple vibes that is the main theme thus far, so you need to consider how to get around that.
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Some unrelated issues with the story in general.
- MC, Claire, Zoey just moved into an expensive looking new apartment even though all 3 of them are unemployed. How did this happen? It seems like none of them even had a career to begin with, so it's hard to believe they had a huge nest egg saved up either.
- This whole jobless trio situation is so weird man. They are so relaxed and chill. Claire is just hanging out in the gym. MC and Zoey are lazying around at home. The conversation of MC and Zoey at the cafe is just so far removed from the reality of their situation. They are acting like work is just something they could look into to kill some time lol. "Oh, why not photography, sounds fun right?" "What about programming? You can code some stuff amirite?" "You are pretty good at drawing, so why don't you get a job drawing things?" Meanwhile, I'm just over here wondering how they are paying their bills.
- MC gets hired as a relationship blog writer... for some random lady who runs a relationship blog... before she even checks if he can actually write shit that is interesting... which has pretty much nothing to do with programming. None of this even really makes sense.