Professor Cha and his

I saw someone speculate that Wooju's ex was the professor's "sex partner", but isn't he married? Moreover I didn't see any wedding ring on his finger so idk. I'm kinda invested on the professor's potential partner here. How come Yeonu knows the ML but he doesn't? And Prof. Cha's expression when he saw Yeonu is very sus to me. XD I am more invested now on the potential second couple.