If you think about it, organs that serve dual purpose for excretion and sex is disgusting. You're doing oral where pee also comes out. Most people don't think this is terrible because speciesm. We think possessing anus hanging over one's face is disgusting. But if snails could talk, they would say it is disgusting to have sex where your organs also just peed.
but poo is actually nasty, it has pathogens and is intensely revolting to most people
whereas pee is fairly inoffensive and mostly just becomes gross by association with gross people or by sitting around for days on end so bacteria can fester in it
i guess because the urethra is prone to urinary tract infections so it's kept away from the bacteria-expunger and put with the Goal Of All Evolution device
Urine probably also helps flush things out too, so you don't need to deal with as many infections from bacteria who also want a share of the nutrient slime gametes get.