MC wanter sex after that date, boss probably just thought of a quick way to refuse, probably should have given a different answer but i guess she panicked a bit. Those 5 years of feelings won't be easy to shake off if she confessed now. That or boss is also just playing around now or testing him
Why hate Gotou? MC basically wanted to fug her after first date just like that and her having a bf is an obvious lie, she wouldn't have a date with him like that if she has already a man.