You make it sound like you do it a lot. [Though knowing Oikawa's stamina he wouldn't be surprised at this point.]

I never really did it often, just sometimes when I woke up early or when I was tired of homework just. [He licks his lips, mouth dry.] It was more to get rid of frustration?

[Unlike this. He prefers this, having Oikawa with him even if he ends up flush against him when soap lather all over his skin.]