I thought the actual episode was average. The one funny line I thought was "If every kid needs to see this movie, why don't you release it for free, on the internet?"
But the last part...still funny. Not just the song.
The song (NSFW!) http://www.southparkstudios.com/clip...t-in-san-diego
While that's hilarious, it's funny (IMO) because of the setup during the episode.
In case you missed it, before the first commercial Kyle says to Stan "Just be careful you don't end up naked and jackin' it in San Diego." then walks away And Stan says "What the hell does that mean?" Then before the second commercial Kyle says "But when you're naked and jackin' it in San Diego, don't ask me for help." then leaves again. And Stan says "Why does he keep saying that?"