Harem I feel is justified given how people tend to seek connections when they are in times of distress, and everyone is in distress
So far, we've established the model slept with him because she doesn't care who she sleeps with because of her marriage, his teacher is using him to get herself pregnant and fill the void in her life because of it, the bully his probably gay but can't come out and say that to MC because of all the terrible things he has down, so instead continues the cycle of violence so he can control him, and Chako, who we know cares a lot for MC and lacks any emotional support with anyone else in her life, would probably feel drawn to him.
Hence why he's in the "abyss" in that he has all these relationships that are shallow where they project onto him their insecurities, which pulls everyone down with him and perpetuates a cycle of reinforcing behavior.