The game is BSS Netorare. BSS stands for ぼくがさきにすきだったのに / Boku ga Saki ni Suki datta no ni loosely translates to "Even though I loved her first" which is a subgenre of NTR where the main guy and girl aren't officially in relationships such as girlfriend or wife, but the story still evokes feelings of jealously and loss when the main girl ends up with someone else.

It seems you don't know the difference between NTR and cheating. Any high-level weeb knows that NTR is when a girl MC likes (it doesn't matter who she is - a childhood friend, blood relative, or just some random girl he had a crush on - she can even don't know him) have sex with someone else. That is it. And if she eventually betrays MC and enjoys time spend with the other guy, then it is high-tier NTR. The point of NTR is to provoke the dark emotions of jealousy and betrayal. It is not about being cheated on in relationships in particular. It is different from c*ckold (why the hell this word is censored?) and cheating genres, it is more broad.

Hell, this game has NTR tag on dlsite, the original publisher of Japanese version.

so the characters depicted in the story doesn't even need to be in a relationship for it to work, they can be crushes, family members or anything relatively precious to the character having that other person being "stolen away" even though it was not theirs to begin with. Self NTR also exist where it's told from the perspective of the person being stolen away where they feel guilt for "betraying" the person they love even if the other person may not love them back.