Shūzō Oshimi... oh how your works give off such a despairing aura. Of a hope that will never come true!
It should be obvious at this point. Any reader of his should know. It's so obvious that it feels a little dumb writing about it.
Aku no Hana isn't about generic base desires or perversions at all. Happiness isn't about vampires, and The Drifting Net Cafe isn't about... whatever.
Actually, they all share a theme. Of some mysterious girl who gives the feeling that they know the main character, and who unconditionally loves him for whatever reason.
And he ends up fucking up his life just for this girl. Is she even real? The way shes written in each of them is as if she's just someone in his head.

I'll just get into Aku no Hana then...

Nakamura is the girl in his head. What she wants is for him to accept something really important.
The first half is about him trying to reach the ideal of becoming Nakamura. But he realizes it's impossible.
The two of them end up trying to kill themselves but she pushes him away before he can die because she couldn't die in the first place, and a guy like him never had the guts to kill himself either.
In the second half, she's gone. Coincidentally, his character has longer hair now, almost like a girl. He's totally lost in society.
His life has totally gone gray. But he finds a girl who likes the books that he read when he was younger, and that brings back the feelings that he had in the first half.
So he ends up finding Nakamura again. Who is living in her own world, watching the sunset everyday, having no sense of time. And for a moment he's finally happy as he says to her, "things are fine as long as you still exist." Then she says he's become a normal person, and he ends up living a normal life and marrying his gf.
Things end with him waking up in a sweat after having that same dark dream that he's always had...
Where the flower of evil he sees looks kind of like Nakamura.
Anyways, if that wasn't enough, he wrote this at the end of Inside Mari's first volume.