Why do we use "retarded" as a denigrating term but not "gay"?

I realize that using the word "gay" as a derogatory adjective is rarely done here, and it's recognized as wrong. But I guess I wanna know why we use "retarded" in the same context A LOT and no one seems to mind. Is there a difference? Why or why not? If saying "that's so retarded" is as bad as saying "that's so gay" then why do we do the former and not the latter?

EDIT: i mainly meant "we" as redditors because I hear "dude, that's fucking gay!" a lot IRL, but I rarely see it on reddit. And if I do see it, it's usually on its way to being downvoted into oblivion, or it's a joke.