The manga that asks "is it ok to bang your student if the student is an adult?!"
I mean, it's still a little weird and/or creepy. Even as consenting adults it would be unprofessional. It's a weird situation that doesn't suspend the student-teacher professional boundary. The creepy part is that he set a very clear boundary but she doesn't seem to care.
Then again, she's not really a student. Her grades don't matter; she won't affect her career if she flunks or quits. So he doesn't really have consent-voiding authority over her. This is an absurd what-if scenario where a not-student gets to bang their teacher, and it's (big air quotes) "ok." Still looks weird.