Okay, I'm looking at these answers and I see potential. Except I know the real answer. First, we have to understand what pee is. Except we already know what pee is so we can skip this part.
Next, we have to understand how this pee gets there. Liquids enter the body and are excreted later in the body, or something like that. Basically, it's because you keep giving your Pokemon Fresh Water instead of Super Potion, you cheap sonuva.
Now we have to understand the difference between boys and girls. Boys are big strong and muscular, therefore they can withstand peeing standing up. Girls, however, are genetically weaker and must sit down. contrary to popular belief there is no bodily difference between boys and girls. You're either a sissy or not.
What about genderless? Well genderless just piss oil through their robot genitals and those legendary Pokemon aint got a fk in the world to give to go and piss.
So your answer is robo oil piss and nobody cares.