It seems as though people think that the dad knew everything that was going on and that the girl is mean towards Shirotani but it's not like that. If he did know I doubt he would just leave his son alone considering he's seen him act differently and hurt. It's just a case of dramatic irony and blaming the father's character for not genuinely knowing is a bit redundant. Dramatic Irony people, it's a common story troupe, seriously.
Because he is so stupid that he doesnt realise what that girl has done to his son.
You want an important plot devise to be solved that easily?
The dad is having a relationship with a *student* who is not at all emotionally mature enough to care about anyone but herself, and he obviously doesn't know her very well at all. He's is not paying attention to how his son reacts to her or how he reacts when the idea of a new mom is brought up. The reaction is extreme--any normal person would talk to the kid about it, and maybe to the girl.
The girl is horrible, but many teen girls are. The dad is the adult who should know better, if he cared to pay attention.
I think people just don't have sympathy for pervs with bad taste in women.
If his dad hadn't been banging an underage psycho, then Shiro might not have ended up with such serious issues.
Then again, if he wasn't all mental, Shiro wouldn't have ended up in such a wacked out relationship and we wouldn't all be hooked on this manga...
I don't know if I've missed something or so, but to me his dad is a carrying father who would like to have a relationship with someone who also cares about his son. But unfortunately the girl he chose is a bitch