Interesting title.
When your Sensei tells you to get naked to fuck, you don't just tell her that you'll "pass", you know? I guess I can respect the fact that he's devoted to his girlfriend now though. Heh, but you don't just snub a horny ero manga artist though, lest you find something slipped into your coffee.
That ending though. Wait...Does that mean he actually decided to stay faithful...To some extent after what happened and answer the phone instead of spending the night? Either that, or it's one of those, answers the phone while having sex, or answer the phone just to say that he'll be busy helping his Sensei...