Classic NTR story. Boy confesses to girl, girl reveals that she's been "training" with her boss, girl fucks her boss in front of her new boyfriend, girlfriend discovers that her boyfriend has a smaller penis than her boss and girl promises to continue her "training". Meanwhile, the boyfriend just sits there and takes it.
Porn before plot and porn before realism. Not that that's necessarily a bad thing though.
It's hentai, you know... the genre that includes girls being raped by octopi. Realism isn't always the goal, sometimes realism is what it's supposed to be an escape from.