I wouldn't completely agree with you, but you've got a point here, I guess (since you know how it works). In this case Shirotani's change of attitude was due to the trauma, so it was rather sudden. But let's say the father couldn't notice that.
The main problem is that Shirotani's father declares he wants a mother figure for his son and then... he chooses a high school girl, who could be Shiro's sister, but not his mother -__- (I won't even comment on that abuse of power). He's just a hypocrite.
Maybe it's just me, but being able to brag about your parent's beauty or young age is not that important.
Maybe--but the father knew the girl was underage and his student. This makes him a bad teacher and parent. Besides, clearly he didn't do much in the way of caring who was with his son.
In the story they have a sudden onset of OCD caused by a traumatic event. In real life, it probably has biological as well as emotional components. A descent parent would notice the behavior we all saw in the story.
I think that what we are starting to see—as I suspect Kurose has already seen—is that Shirotani has self-diagnosed and identified himself with the mysophobia disorder, and his diagnosis isn't completely accurate. His OCD behaviour is symptomatic of someone with mysophobia, but it masks a different and larger dissociative problem with sexual guilt and shaming triggers. Bipolar, probably.
I think you may be onto something. Personally, I tend to think both OCD and mood disorders have a biological basis, though they can be triggered by stressful events. This story tends to be old school Freud, and it involves a child not being able to make sense of events or his body's drives, so he pushes it into his subconscious and it is expressed as mysophobia.
Take into consideration that the father did not know of the high school girls intentions, and that it probably wasn't serious.
In all honesty, the father perhaps assumed the girl was kind, and never saw her two faced character or goals.
So him picking her as a replacement mother, or even considering it, wouldn't be completely out of the question, since to him, she's not who she seems.
But I doubt he'd choose her. He just seems to sate her as much she does him.
Man with needs can go elsewhere to release his urges. Meet a woman his age, or at least older than 18, and have a normal relationship ╮(￣▽￣)╭
People are going off on the father, and as a daughter with mysophobia and OCD tendencies, it's rather difficult for them to understand the root of the issue.
For me, it's weight related. I see calories and trash on everything. I obsessively observe people and what they touch, to see if they washed their hands before touching something else.
When my family started noticing my ocd and phobia, all they could do was accept it, and hope that I myself get treatment (forcing it isn't an option for people with this condition).
The father is doing the best he can as a single father and man with needs. All he can do is hope to help his son, as much as he can, while also being human himself.
Not to forget, Shirotani would never admit to the source of his phobia and OCD, since it is rather twisted.