Here's how I see something like that happening;
>Episode starts with the boys hanging out, then Cartman comes in and goes on about how awesome MLP is.
>The boys are skeptical at first, but then they start to like the show too, with Butters, Kenny, and the minor characters starting to like it too.
>Their fathers find out about their new fandom, and at first they are horrified. Then they become bronies, after watching an episode (depicted in a satirical way.)
>Stan and Kyle are the only non-bronies left, then Kyle converts slowly, after watching more episodes.
>Cartman makes some really creepy-ass fanfiction and fanart.
>Kyle tries to convert Stan in a friendly way, then his dad shoves it in his face.
>The girls are annoyed by a new surge of male fans, but slowly accept it.
>Stan goes to the source of it, Lauren Faust.
>Lauren Faust and Studio B are actually crab people. When Stan realizes this, he tries to kill her, but is seized by bronies.
>They tell him that MLP is the best thing for the planet, and the crab people bring up images of pony to convince him so. However, Cartman's (and many other basement dwellers') fan art shows up instead, showing a bunch of grotesque and horrifying pony art.
>Everyone becomes disgusted.
>They still need something to feel hope, so they turn to the Sonic fanbase, ironically.

But that's just me…