South Park Episode 307 - "Cat Orgy"

Shelly Marsh is babysitting Cartman.

Shelly invites her boyfriend over and he in turn invites his band. Cartman has to prove that Shelly has violated the rules. Meanwhile, the Cartman's cat is in heat, searches for a good time and invites all the cats in the neighborhood back to the empty Cartman house for an orgy.