Everything in this manga is for comedy purposes but there is a plot other then that which is the prison itself. And BlackJack thats a major spoiler man, thats why Hana is best girl. Not to mention that was a big character development as she realized that its not for revenge as she has feeling for the MC.
As for the peeing part the MC saw her pee first by accident, peed on her by accident, almost fucked her by accident which lead to everything Hana did (she doesn't really have a pee fetish). Everything lead to them having their first kiss (both of them liked it), then that lead to the face photos and the realization of Hana that she has feelings for Kiyoshi (good thing that short girl with the glasses showed up cuz it was going into straight hentai) as she get incredible jealous for the Uroboros moment in the bathroom between Kiyoshi and Mira with the snakes, this lead to the recent moment in the manga with the 69 scene which was hilarious and they shared their second kiss.
The only fetishist in the manga is that big fat dude Andre being a masochist and later in the series Risa, no one else has a fetish (the counselor doesn't count as he is the most badass Ass-Man there is).
Also Anzo watch it, don't read stupid previews as they are mostly wrong, the anime is incredibly funny. You will be rolling on the ground laughing.