It's mostly conjecture, but it's because of the Flies story in 'Something everyone knows and ignores':
So, the story involves a male who hid his sexually because of his parents, became twisted, decided to crush flowers so others couldn't enjoy them, then killed himself.
Then there's Nozomu, who seems to be both Callous, and Dead, and seeing as how religion seems to play a big part with Saki:
Who is likely his and Sensei's mother, I can see Nozomu's parents forbidding him from being gay, which caused him to become callous.
Of course, I could be way off.