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If you've read the prequels and the sequels, you'll see that Kuon had been watching Nanao and semi obsessed with him for the entire time he's known him, which is at least a year before Nanao coerces him into sex for the first time. Since Kuon always thought Nanao was lovely, he was so disappointed when he heard the rumors to the extent that he felt betrayed by Nanao, which is why he disliked him so much at first. In actuality, Kuon is an obsessive personality who at the heart of it always wanted Nanao.
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Exactly, it's the only reason Nanao can 'coerce' Kuon into sex, when a woman can't even lay her hand on Kuon's without him saying something and stopping her.
This manga was kind of odd in the since that I didn't feel the love that he had for this person he barely knew. Yes he was wrong about his personality, but he learned nothing about Anao besides that fact that he had a crush on him for a very long time. I think I stead of saying I love you, he should have said Im sorry for the rumors and I would like to build on our relationship since I do like you for no real good reason besides feeling guilty and loving his touch which can be felt with many people since that's called lust. Love the aurther other work though. :)