So the Sensei liked him too but was afraid to take a step forward because he was afraid of...

So the Sensei liked him too but was afraid to take a step forward because he was afraid of being forgotten so that's why wen mio approached him,he only agreed to hve physical relationship knowing that it was more then enough, he was happy which mio thought the same too.
What i see here is communication problem from Sensei 's side but i kind of understand him too because I'm the same as him.
Thus him acting possessive was to make sure mio doesn't forget him.
So cute.
I love it♡