Nah, the tag is correct and needs to stay.
Why? Because Stefany very much did NTR to Oliver back when she was pushing him as hard as she could co she could suck and fuck Arnold any time she wished. NTR is when a partner of the person affected willingly does acts that they know will cause emotional impact and distress on their partner. Stefany did this at least twice in the beginning; 1) when she hid what she was doing but very clearly appeared (did) a BJ on Arnold in the tub when washing him. 2) When Oliver was looking through the keyhole Stefany deliberately went well beyond what they agreed she would do when she and Oliver talked about it before, such as (depending on who you ask( either faking, or having full blown fucking with Arnold well before they said it was ok.
Now some will downplay what Stefany did, saying that it was all agreed on beforehand but it wasn't. The supposedly faux fucking scene for example, and I say faux (fake) because Oliver himself *knows* she is really fucking Arnold although he doesn't want to admit to himself that she is. He alas, like a coward, refuses to actually confirm with her if she did or not after the fact. But as written in the story she was expressing exactly the same sounds and body movements that she does when fucked so it was clear Arnold was fucking her, unprotected, well before their 'approved' episode when they come back from the boss.
Basically the NTR here is that Stefany was deliberately doing things with Arnold, including BJ and sex, before they agreed she could go that far, for the simple purpose to hurt Oliver and make him go beyond his comfort zone so he would ultimately agree to her wishes to fuck anyone she wants. If you want to say "but Oliver wanted it!" I say go back and actually read the story of what Oliver is saying to himself and the pain he felt in his chest at seeing the (supposedly) fake fucking she was doing. He very much was in shock and pain that she did that without approval or asking, and thay pretty much broke whatever resistance he had to her fucking and she took the full reigns of the relationship at that time and the story has never looked back.
Stefany is really the MC of the story, until the dev tried to retcon in the last few releases and have it more balanced, but she very much did NTR against her supposedly loved husband, to get to a place where she wanted to be. The dev may write more comments form her recently that she loves Oliver and is only doing things for him, but really she's just grooming him and keeping him pacified so he doesn't realize what's going on and reject it. Same thing with her getting more women for him and 'allowing' him to have more sexual relations. She's controlling the show and is not doing NTR now because she doesn't need to, Oliver is broken and she can do what she wants because he accts her as a slut and likes it, when that wasn't at all what he wanted in the beginning when they talked about it. It was to be very controlled and only when they both agreed to a couples or swapping/swinging idea.
So the tag fits and was correctly applied and should stay.
thats all this game is missing is Stef getting fucked in the ass by Arnolds dick as hes fuckin her pussy with his lucky cane
