South Park (1997)

Season 1: Episode 4

Stan's got a new dog named Sparky. Stan proclaims his pet to be the toughest dog on the mountain. Though while Stan thinks his dog is ready for fighting, Sparky proceeds to hump Sylvester like a little bitch. The other boys taunt Stan about his gay dog and Sparky runs away to find someone who will love him for who he is.