My [F27] boyfriend [M30] is a tub butt

My boyfriend is a Tub Butt, something I've told him repeatedly I find gross, and he insists it's normal. We have been together for 4 years, and living together for almost 1.

What is a tub butt you may ask? Well, our toilet is within reach of our bathtub, which our shower goes into. We take at-least daily showers, so there's usually a bit of standing water in the tub while it evaporates from the last person's shower. After taking a particularly messy poo, my boyfriend insists it is normal to dip a piece of toilet paper into this standing tub water, and then wipe his butt with it. To make matters worse, well, we're not exactly clean freaks. The tub under the shower is not cleaned except for maybe once every few months.

He insists it's better than the alternative (of... what? just wiping your butt more?) and refuses to give it up. I think it's gross and now call him a Tub Butt. My boyfriend thinks I'm being too harsh. Am I the crazy one here?

tl;dr my boyfriend wipes his butt after dipping toilet paper into the water leftover from the shower. am i crazy for thinking it's disgusting?