I mean, we're talking about people who fundamentally don't understand what cucking or NTR is... Dudes who are incorrectly labeling scenes in Intertwined as Cucking/NTR because they're upset Erynn has gotten other dick in her lifetime. One of the dudes complaining was literally upset that Erynn didn't save her virginity for the MC... I really really doubt Nyx is writing the story with the skewed viewpoints of idiots like that in mind.
It's just a simple blackmail situation to create some drama in the story and to show how much of an asshole Nathan is. That's literally all it is. MC is upset in CH10 not because he's watching a video of his childhood crush fucking her boyfriend (because that interpretation is beyond ridiculous), He's upset because he found out that Nathan wasn't actually bluffing about the video and that Erynn's fears were true. He's upset because he cares about his childhood friend and she's dealing with some very real shit, not because she *gasp* fucked her boyfriend.