He was drunk and Im pretty sure that’s why he forgot he did that to him.
idk if the timeline would actually align but my interpretation is akamine was probably fantasizing about having sex with saeki when he did that to utsumi. remember that one flashback scene where it showed akamine's desire and it was him and saeki having sex? idk if it would make sense but that's how i tried to think the reason why akamine did that to utsumi. which makes it tied in with the fact that he couldn't even remember doing it in the first place and forgot about utsumi since it's like just a dream for him. this does not make his actions justified however. what akamine did is sti awful
why did he rape him in the beginning though? the whole time i was waiting for even a hint as to why he did it, but no. shits and giggles? surpressed gayness? or is it like representative of rape being arbitrary?