This pejorative use doesn't actually seem as common as I remember!
Looks like there was a bit of a decline after Season 7 (2003 or so) but when there are spikes in usage it is usually narrowly confined to a few episodes in the season. Here are the counts:
This measurement, based on crude word counts, isn't perfect because I'm just looking for the string "gay" and not looking at all at the context in which it is used.
But it does (correctly, I think) show that in Season 15 the word was not used at all!
(All the hard work here is due to the fine folks atMore questions on .)South Park (TV series):