If MC says YES, Yandere-chan will cling to him forever, dealing with his B-cup girlfriend at some point down the line.
If MC says NO, Yandere-chan will cling to him for a day,
then probably kill his B-cup girlfriend, then return to console him.
If MC tries to NIGERUNDAYO out of there,
Yandere-chan will track him down, kidnap him, and live "happily" ever after together, far away from his B-cup girlfriend.
The lesson here? If a Yandere already has her sights set on a target, it's probably already too late.
@Vini Basically, it's a harem MC who decided to date the girl with the smallest boobs so that all the girls will stop arguing and would instead push him into their boobs and lets him bang them since they feel they have a good chance of stealing him from his girlfriend and forgetting about all other rivals...