What you described is still NTR, bro; just because Dylan wouldn't know wouldn't change the genre of the scene, just less extreme than what happened.
Also when it comes to Swinging which seems to be what you keep defending, it was never going to work and it would've been always considered NTR because this story started pretending to be an Incest story (mom-son), but as it turns out the autor wanted to just be a better looking Pigking, chapter 9-10 killed this story, so many people back from 8muses left. You put something in between the mother and son; it's NTR, worse, it's the worst kind, mom NTR.
If Pegasus wanted to make this story Swinging and not NTR, he should've started from the beginning, but then no incest crowd would've gotten in; instead, he spent 7 chapters developing his protagonists incestual relationship, just to have the female MC railed by someone else in front of the male MC. That is like a 9/10 on the scale of NTR; your idea just knocks it down to a 7. Dylan not knowing would be a bomb that would explode even worse in the future too.
Pegasus should've wrapped this story of Brandi-Dylan the moment the mother character accepted to have a relationship with her son, then made some epilogue with her pregnant, not even implying sharing; maybe add the aunt if you want to be a completionist; it would've been perfect.
IF he wanted full swinging he should've used a new set of chars, that could've been introduced with Dylan-Brandi and took the mantle of MCs from them to keep the Title going, like Victoria or the other useless characters, then they could START with the swinging, not with the incest, so the NTR would be lighter, much lighter. But yeah, if Pegassus had done this with Brandi, the story simply wouldn't have been as popular back then.
Anyway, I'm only here for morbid curiosity, though I'm happy I'm not the only one. I remember
@Gnaparelli from way back when this was good.
@PPanGames I didn't notice your cut before, since I only maintained a minimum attention span here and only delved into the comments when making one, but from what you wrote, you did what Pegasus should've done: isolated the protagonists from the filler and unwanted developments. I will check it out and hit you back.