Liane Cartman is both Eric's mother and father. How is that? She's a hermaphrodite, of course! Liane is also infamous for being a bit of a slut around South Park.
Chief Running Water: (Narrating the flashback) I knew she wanted me because she kept saying romantic things.
Young Liane: Oh, Chief. I want your hot man chowder.
Young Chief: Whoa! Hello!
Cartman: No kitty, this is my pot-pie.
Cartman: No KITTY, THIS IS MY POT-PIE! MOM! Kitty's being a dildo!
Liane Cartman: Well then, I know a certain kitty-kitty whose sleeping with mommy tonight.