It’s not so much the existence of the thing - the not-a-bathroom-bathroom - that confuses him as it is the fact that he - plain-faced, shaggy-haired, awkward, cannot-talk-to-girls Midoriya - is standing in it.

Tsuyu takes charge of the moment. Midoriya doesn't know what to make of that.

Midoriya still doesn’t understand girls, but he sure as hell learns the meaning of the word ‘orgasm’.