The use of "gay" as a pejorative is not merely a matter of carelessness in most cases. Although the action has not been thought through in many (perhaps most) cases, the word, since its current primary meaning is that of "homosexual," has a periphrastic connotation of homosexuality.
Thus, even if the word is being used in its sense of "oddly different," it breeds a very strong unconscious association between the meaning of "bad" and "homosexual."
I can attest to this happening among young people myself, as I am a straight but not narrow high schooler who recently came to school with my nails painted. What I had done was summarily called "gay" in its senses of both "homosexual" and "bad," where both meanings were clearly intended.