Well sometimes to avoid remembering traumas as "traumas", we unconsciously try to make them seem less important or else it will break us apart, and I think both of them are unconsciously lying to themselves to feel less pain, that's why they made fun of it and even made it into a challenge to laugh it off. They are basically pitiful idiots trying to be happy in a shity life.
But Kuzuya kept saying how traumatic it was. So he's not in a denial, he doesn't try to make it less important because he's perfectly aware of how traumatic this whole thing was. He even asked him to stay with him after work because he's clearly scared and doesn't want it to happen again. But Katsuya literally doesn't give a shit. He even felt envious when he saw Kuzuya being raped right in front of his eyes or when he told him how shitty he felt about his situation and his past... So... ╮( ̄▽ ̄)╭
I love harada, but kasuya is highly unlikable and one-dimensional. I just can't with his reaction to what happened to kuzuya.. Feels detached from basic human feelings. Except if Kasuya is an actual psychopath and can't feel empathetic.. (though that should be focused on) You can't put actual rape in the plot and make it funny/ don't explain why he doesn't feel for the victim. What the fuck! Lacks depth so far. Maybe it gets better as the story is progressing.