Now, open the door!

In his drunken stupor, returning student Moon Yeon-su ended up accidentally at his downstairs neighbor’s apartment, having the same password as he used instead of his own. While there, he saw a coupling/union (?) between the Yoon Do-a, teaching assistant in his department, and Hong I-jae, a fellow student.
The shock of it all wore off, and the following day, after a great deal of reflection, Yeon-su recognized the seriousness of what he had done (?).
“WTF!—that was a pervert yesterday?!?” However, he couldn’t get the enjoyment from last night out of his system, and he ran into them both again, which initiated a turbulent and raunchy college existence for Yeon-su!