Ι've gotta say though, given what I've glanced from the comments, y'all are dealing the boss a really shit hand here. Sure, she rejected the MC, and with a huge lie, at that. But the MC was definitely coming on too strong (assuming she's really a virgin at her age, being invited to bed like that is not the kind of offer one would take casually) and, if anything, this disproves what people keep shouting about her "manipulative" nature. If she really was manipulating him, wouldn't she have found a more plausible reason to reject his advances, convincing him that he's still in with a chance rather than shooting him down flat?
That's one. Second, I really think people are blowing the whole "wants him because somebody else got to him" thing out of proportion. Remember, she first started moving on him BEFORE he rejected the consecutive business trips and before all these signs she brought up (going home early, checking his phone a lot) became this pronounced. She might have felt that the kouhai was getting closer to him and forced her to move faster, but that's perfectly normal when you realise you like someone. I don't think she's the best option for the MC right now, and I definitely believe he's gonna end up outgrowing his crush on her before the story moves on to the finish, (whatever that may be) but I doubt she's as horrible as y'all try to make her out to be.