The problem isn't the other girls are there. Its is that the long burn is too excessive for some and the pacing is just terrible. Especially with the main girl. The only one done right is Melissa, main girl's best friend. She goes from a blow scene, to full on vaginal scene, to anal and more for all route. She is paced good. Her story decently flesh out, and she has motive, mind you this is a side character. She gives into the NTR. She feels more completed then the main girl. The main girl has only the story to help her, but the long burn is the issues. Too much tease. All foreplays and orals. And by the end there is absolutely no reward for her. She does not give into the whole point of this game. The NTR.
Only the blackmail has her doing anal but its FORCED. So from this POV, its not NTR but raped. She does not give in to the rape either. No NTR reward. The cheating route she clearly cheated with fore play, kissing and touching but games refuse to let her give into NTR. Both through dialogue and actions, thus again lies the issues, no NTR reward. Cuckold as you just stated, no rewards. Just a bunch of random pointless things that I don't even know if you can considered it cuck, let alone NTR? After 6 hours running through it, I was like um where is the cuck part? lol. Huh?
The point is, the side girls characters should of gone from one fore play or oral on first encounter, then straight action on second encounter, by the end gives into the NTR, while the main girl should of get the full from beginning to end treatment for the chapter, start with dialogue and motive around first act, kissing, foreplay and orals around second act, soft action like skin contact and outside vaginal and penis rubbing around third act, and full on vaginal penetration by the last act. The damn game is like 8 hours if you read everything and try to do all side quest. Where's the NTR!