Yeah, but are they doing it in the house, or outside in your game?
I have more dogs then cats in my game, because I was breeding some dogs.
The dogs in the game will get bubbles over their heads if they need to go, and you can just send them to do it outside.
My Sims have accidentally stepped in piles outside, and get the grossed out mood, but afterwards they can just clean it up, and wash their hands.
Cats have litter boxes you can buy for them that just zaps the waste away, or your sims can just scoop it.
People want realistic play, so realistically your pets need to poop, just like the Sims.