I agree he's a dick, but how does a teenage boy deal with sexual expression? In the US, these emotions are usually portrayed as getting a girl drunk and raping her.

Someone here said Japan is not afraid to tackle uncomfortable and or forbidden situations. I agree 💯 percent.

Being a dick is not an unnatural response to extreme psychological confusion. It seems to me that many commenters here are either disappointed that there's not a lot of titillation in the film. Or they find even the idea of experiences portrayed here unnatural.

I found the film very realistic though it might have been more clearly portrayed in a dorama with more time for the viewer to tackle these taboo subjects.

Perhaps I understand more because I speak from similar (not identical) personal experience with incest.

As the movie progressed I recalled an article or discussion on twin obstetrics. Twins are naked and in extremely close contact for the first 9 months of life. There have even been cases of C-section births where the siblings were found to be in a close embrace. Toddler twins can go through emotional distress when parted.