I think a big part of the reason why Kyousuke told Kasumi he was mistaken is because of his view on love and happiness. From a younger age he was taught that liking boys was wrong because it was not normal. When he became someone Kasumi could lean on I don’t think he said it because he didn’t believe in Kasumi’s feeling, but that he must I’ve done something to trick him into thinking that. He felt that was about Kasumi and, as he said, he was afraid of corrupting him. Corruption as in influencing/leading him to the way of un normal love and therefore not real happiness. Kasumi wrote that he wanted to get married and live happily and Kyousuke didn’t think that was possible from being in a relationship between two boys. He felt that he was already on the path of ”corruption”, but Kasumi still had chance to live a ”normal” life by being with a girl.
In short I think the statement came from that Kyousuke always believed deep down that a future with him would always be a worse life (because it wasn’t ”normal”) and that by tying him down by being in a relationship with him he was robbing him of what he himself saw as living happily. As he said to Kasumi, if you’re going to be unhappy if rather you be unhappy with me - instead of saying he’d make him happy because he didn’t believe it was something he could do. He wanted Kasumi to live happily because he loved him and because it had been his dream since they were kids even before they met. But Kasumi’s view on happiness was always centered around Kyousuke by being with a girl he was following Kyousuke’s view on ”normal” happiness and not his own. As Kyousuke stated, he was sorry for hurting Kasumi because he was a coward. Of course that doesn’t make up for it or make ot right, but when he said Kasumi was mistaken I think Kasumi interpreted it as Kyousuke not believing his feelings were real love and Kyousuke believed the feelings of love were real but that the only reason why Kasumi would feel that was was because Kyousuke had ”corrupted” him.
My god do I hate characters that invalidate the other person’s feelings like “you’re mistaken it’s not real” and even worse when they’re arrogant enough to decide for the other person what’s best for them and what will make them happy, nothing more I hate than taking the choic3 away from them,
bro are you serious? Making the same mistake twice then and now, after confessing and crossing that line and sleeping with him you’re just gonna easily let him go thinking your feelings weren’t real and you did it out of pity? Instead of holding him back and making him realize you love him and promising to stay by his side you just let him leave again? “For his sake”? Thinking he’d go have some happy life with some girl? Was the first time of seeing him be so broken he chased after a married woman he didn’t even love just to lick each others wounds not enough evidence that the future you imagine him to have isn’t gonna happen just cause you push him away? Like he’s so goddamn wishy washy about it all, promises everything to him and the next moment pushes away “for his sake”, then pulls him in again only to do the same again… if it was for his sake he’d stick t0 him and promise to make him happy the second time and not leave it for a damn third time