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.

Liane Quotes

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!

Kitty: Meow.
Cartman: No kitty, this is my pot-pie.
Kitty: Meow.
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.
Cartman: What?

South Park Quotes

It's a man's obligation to stick his boneration in a woman's separation, this sort of penetration will increase the population of the younger generation.

</i> Cartman

Cartman: Suck my balls.
Ms. Choksondik: Present them.
Cartman: What?
Ms. Choksondik: Present: Whip them out and I'll suck 'em.