At page 3, the Japanese text reads "お湯のワキ" (oyu no waki) and Nozomi has to write the kanji character for "ワキ" or "waki" which means "heat (water) up" in this context. Nozomi instead writes the kanji character "腋" which means "armpit", hence Tsuyuno calls him a pervert.
Now it's over, for real...