Great....

Classic fujo behaviour. You would've had no problem with the so-called "teacher-student" dynamic if only those two had been the main couple. A rare instance where the garbage dump that is the romance genre manages to come up with a nominally "teacher-student" sexual relationship that is actually a non-problematic agreement between two consenting adults, who are independent of each other and equal in social hierarchy is the moment when their mental asylum escapee audience who slobber all over insane pedo/groomer/incest shit suddenly picks up the book on morality and starts coming up with moral problems where none exists. Just admit that you don't like the character because he is antagonistic to your protags. You don't need to gaslight yourself and other people who come across your comment into thinking that you are this deep, morally upright, responsible person who has a good, serious reason for hating the character.

It's been clearly indicated that both the ML and his ex were adults when they got involved and that both of them were close in age. I only skimmed through a few chapters here and there to gauge where the story was worth my time or not and even I know that the ex is only his teacher as much as your gym trainer only a couple of years older than you would be a "teacher" to you. Your school teachers/college professors when you are a minor/barely young adult and your instructors at the various hobbyist classes you join as a complete fully grown adult aren't the same when it comes to the nature of the relationship you share with them respectively. In the first case, they are people you look up to as your mentor, a guide, they are supposed to be your caretakers. In the latter case, the instructor is your peer, in every sense. The relationship between the ML and his ex is the latter one. Completely green, no issues. But, that's the one that you people have a problem with. What you people love instead is when the prior one where there actually exists a power gap, a hierarchy gap and a fucking big fat red line of the dynamic being an extension of guardian-ward relationship is the relationship that gets sexualised by disgusting authors in this genre.

Please go and lecture the people responsible for the scourge that has fallen upon this site as a whole, especially the manhwas, where it's become bloody near responsible to come across a single romance manga that doesn't involve a character and a pseudo-adoptive family figure/a childhood friend or acquaintance who was like a brother or a guardian/a mentor/a superior who is not only above them in power but also significantly senior to them in age hierarchy. Does your moral compass only work correctly once the author informs you that they have allowed you to hate a character? That was a rhetorical question. You are after all someone who is apparently very comfortable with and delights in reading incest as long as the author or the echo-chamber you come from has deemed it permissible to do so.

Fujos love to harp on how disgusting and trashy shounen harems are. I, honestly, can't see the difference between most harems and most shoujo/josei/BLs anymore. The BL stories you love and the characters you fawn over are problematic and repulsive for the same reasons that harem stories are.