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which part don't you understand?
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Well... the flashback is after the continuation of Volume 4 chapter 22. It is a "split chapter." Doumeki doesn't want to have sex with Yoshida because he feels like he'll end up like his father, and because Yoshida only has sex with people he doesn't care about. So Doumeki wants his relationship with Yoshida to be about more than just sex, he wants it to be about love and intimacy, so he refused to have sex with Yoshida since Yoshida said "It's just sex, right?" Doumeki thought he was just trying to have meaningless sex with him before Yoshida threw Doumeki away like he has done with everyone else.
-Shadow
I don't understand chapter 23?