Dictionaries don't define the pejorative use of gay
Some of them do:
- Dictionary.com: Gay: "awkward, stupid, or bad; lame: This game is really gay.
- Oxford Dictionaries: Gay: "Foolish, stupid, or unimpressive: he thinks the obsession with celebrity is totally gay."
- Wiktionary: Gay (English): "Used to express dislike: lame, uncool, stupid: This game is gay; let’s play a different one. = I dislike this game; let’s play a different one."