This guy literally took advantage of Meguro. Red flag: You don't leave your porno out in the open to check how the girl responds.
I personally think commitment is really important within a relationship. And well, the only thing this guy was committed to was his dick really. He says nice things, but next thing you know is that the guy has sex with Meguro, goes to college and never speaks with her again. There wasn't a proper relationship, it was only a temporary fling. His skewed definition of sex and love is imprinted on Meguro. And Meguro as a result becomes vulnerable, tries to validate love through sex, there are no proper boundaries for her against any guys. I also low-key blame her parents btw by lack of education on relationships.
Even if this "sensei" never technically hurt Meguro, I still think he's a douche by not properly committing to Meguro. Or in other words a dick, because he thinks with his dick.
MC is actually perfect for Meguro because he takes his time to heal her heart. And taking things slow because Meguro doesn't have boundaries is smart for him and her.