I don't actually want the story to be changed so that the MC is a willing participant - that'd be removing the most interesting aspect of it and making it indistinguishable to every other phone-ntr game. Instead, i want him to have a bit more of a backbone when he continuously says he doesn't want this despite still finding it arousing - something more along the lines of 'yeah, I was curious how much i'd like actually indulging in this fetish - and i've had my fun, but this isn't actually what I want. when she steps across the line (which she already has) and i have the chance, i'm gone'. It'd actually be really satisfying to have the GF disappear for a few days and - as a previous poster mentioned - essentially forget the MC, only to come home and find he already moved out and ghosted her. Experience the fallout from the text messages from mutuals and social media posts.
Because the GF doesn't know about his fetish, and seemingly doesn't care either. she blew the 'friend' on the first day and was already making sideways remarks and gaslighting. 'my mouth hurts', 'i thanked him better then you can'. etc. Alongside sexting with him even before that.
The issue isn't that there's zero response to the MC saying no or the MC having the option to do so, it's that the GF has no regard or respect for the MC and his friends are gaslighting him into thinking that's ok.
Basically - The GF being a cheating whore is fine - even hot, in a deeply hatable way - but the part that hurts the story is the MC having absolutely zero self-respect or awareness that he's essentially being used (without his consent) for the GF's fetish rather then the other way around, and their 'friends' are enabling and encouraging it.
Tl;dr - the GF being a terrible person is hot and doesn't actually need to be changed for the narrative, the story needs to actually have characters recognize and respond to that semi-believably.
P.S. Apologies about the rambly message, was editing it to get my point across and appear to have reiterated some where I didn't need to.
This has always been the problem. The sharing kink works but not if the MC does not have sex with her because at that point it is not sharing. And as presented she is an absolutely terrible person. One of the worst people you'd even meet. The MC is so unrealistically spineless