@Richman Weirdly, I didn't get a notification this time
I think everyone's losing sight of the bigger picture.
Gen's motivation is he wants to keep everyone in the town and his "family" together., especially Reiji. Part of it is because he is implied to have feelings for Reiji but can't confess to him both because he knows he is straight but because his father's reputation and the fact he will have to inherit it means he can't come out of the closet. However, he wants Chako to stay as well, not because he has feelings for her, but because he wants all of his childhood friends together and knows that if Reiji stays, he'll need her for morale support.
So what's he gonna do when Reiji cuts town and no one knows where he went and the only lead Gen is able to get from Chako, the person closest to Reiji to his knowledge, tells him about the teacher who forbid her from having a relationship with him and that they haven't had contact since?
We've seen what Gen is capable of doing when he's mad, and if he learns that the teacher basically took sexual advantage of him the night after he tried to commit suicide and has been harassing Reiji and Chako ever since, let's just say things won't end well given the circumstnaces.