Does It Ruin The Yandere Fantasy If The FMC Fights Back?
Not just simply rejecting the yandere verbally or running away, but instead punching him, using a skateboard to knock him out, or shooting him? Besides this action potentially ending the story too quickly should the pair's reaction go too far, it could create a mortal enemies dynamic between them.
Imagine the story potential of seeing a yandere (specifically a black-flag type) getting scarred by his lover and holding a grudge for it (or even deriving sadomasochistic pleasure from it). It'd be even better if his lover participated in his downfall, thus intensifying his obsession. 💕
Though this specific scenario could still end in a variety of ways depending on one's preference. It doesn't even have to lead to this kinda thing. It could also just be a moment of her lashing out due to something horrible he did to or for her. Or, if a dark comedy, it could be a much lighter "Tom and Jerry" dynamic. I'll stop here, since I'm getting away from the question above.
Edit: Since I can't edit the post title, I'll add that I'd also enjoy MM / BL with an MMC that acts this way. Can't recall anything that resembles it though.