I didn't like the first Scars of Summer and I don't like the new one. I can't get the NTR feeling from a game where the protag never has sex, or even (at least) like jerks off thinking about the love interest or something.
Ways it would work for me -
1. They are lovers, she gets stolen, he finds out, and either a. walks away or b. pathetically cucks and turns into a voyeur
or
2. They are just friends/crushing on each other, she whores out, he finds out tries to 'win her back' they have sex...and then he finds out she went back to being a slut behind his back cause he wasn't enough.
To me that is NTR. If she isn't a girlfriend, and you don't do anything sexual, than to me, there is no 'cheating' and cheating sex is like...IDK I'm rambling I wish I wasn't so picky cause like 90% of Japanese NTR games have virgin kissless protagonist.