Discriminations are so irrational, and those behaviours and feelings are boosted especially when they are filled up with paranoia and fear (generally indoctrinated through religions or politics long time enough to become a belief), and so then we have this kind of parents, the kind that even would irrationally discriminate or be afraid of their own children despite the children are innocent in the first place. Some readers might think this kind of parents are ridiculous and would barely exist in reality, but they are actually common in reality, unfortunately common enough that LGBTQ+ children get disowned/abused at a saddening rate all over the world, and parents who act like the father treating MC in this manga are even more common.