From DMP:

Professor Asahina isn’t a very good teacher. Rumor has it that he spends his nights at love hotels with a different partner every night and he has been known to come to class drunk on occasion. When one of his students, Yuasa, finds a film of him having sex with an unknown lady in a love hotel, things get a little sticky. Yuasa promises to help Professor Asahina hunt down the tape and keep his secret safe, but for a price. Yuasa is known to associate with men and has had his eyes on Professor Asahina for a while.