There is no illusion of choice. Beside this one instance, you've had complete agency over which characters you choose to pursue. The thing is, in the case with Mia, you're essentially wanting to be given a choice to reject the story's inciting incident, which simply isn't possible. Because then the game would be just as
GetOutOfMyLaboratory said, and may as well be called "Summer with People". It'd be relegating Mia to being just another potential love interest for the MC like everyone else. But she's so much more than that to him. And there's too much history and unresolved feelings between them for you as the player to be able to just say "nah, I think I'll pass". I mean, that'd kinda undermine and betray the entire premise of the story, wouldn't it? In that sense, why even bother making a sequel to begin with?
No offence, but this would be a terrible idea. If you were to be playing the route where the MC stays faithful to Sadie, then Mia's advances and attempts at seduction would be completely futile because you'd have already committed to the faithful route from the beginning. And if you did want to give in to temptation, then you'd just do so on your "fuck everything that moves" playthrough. So what you'd end up with is a character (Mia) who incessantly keeps trying to hit on the MC because for some fucking reason the bitch just can't take no for an answer. And that'd be pretty annoying and pathetic, don't you think?
Look, I get it. Having the MC forced into being unfaithful feels kinda shitty. But it should. That's life. It's complicated. No one in this game is perfect and they all make mistakes. But how you deal with those mistakes going forward will still very much be up to you, and how the story concludes will be determined by those choices you make.
