And as I thanked the guy above you, I'll thank you too for taking the time to answer me. I'm interested in this VN, I won't deny that, just not a fan of having sharing forced in a monogamous relationship.
Ah, I see what you're asking now. Ok. Mia and the MC are in a casual sexual relationship. They're not monogamous in any way, seeing as MC sleeps with other girls without batting an eye. Also, by the looks of the image above, it seems like disabling NTR removes any scene where you share Mia, but not her friend Emma. MC does some pretty shady things to get her to have sex, though. Even after they're both consenting, MC still resorts to trickery and blackmail to do some sexual acts to Mia. Simply put, he's an unlikable POS. If you're no longer interested in playing the game, here's a spoiler for what happens late in the game.
Eventually, Mia starts to fall in love with the MC. However, the MC has started a sexual relationship with the mom, and starts neglecting Mia. Mia discovers their relationship and gets heartbroken. She spends one "last night" with the MC where she "makes love" to the MC, and the MC starts having a change of heart. But the next morning, she leaves, and writes a letter telling the MC that she loves him, but she needs to leave in order for him to become a great man. The MC ends of up crying.
Honestly, as shitty as the MC is, I thought the ending was quite well done. They have already confirmed that they are working on the script for Summer with Mia 2. The ending to this game sets up the sequel pretty well. Anyway, I think you should have more than enough info to make an informed decision of whether you want to play the game or not. Hope this helps.