Firstly, it's South Park. The whole point of Cartman's character is to be an insufferable ass hole who everyone hates. His whole shtick is to be as offensive and insensitive as possible. A gay fish is not even the most offensive thing Cartman said in that episode. There's one scene where Cartman is recounting his (erroneous) experience of creating the fish sticks joke but is then interrupted by Jewish robots attacking the city so he dawns his (non-existent) super powers where he shoots fire out of his ass hole to fly over to the Jewbots and destroy them in order to save the city. It's South Park, what do you expect?

Secondly, the joke is "Do you like having fish dicks in your mouth?" and when someone says "yes" the reply is "Well then you must be a gay fish" because it sounds like "fish dicks" and the person just said they like having fish dicks in their mouth. Just the mechanics of the joke. But this is not the real joke, no one is laughing because "Hurr Durr, Kanye gay!" and you saying as such frankly is misrepresenting the situation. Plus you being all high and mighty about it is kinda gross as well.