Why do people use "that's so gay" as a insult?

its really lame. I hate it. I live in a phobic community so I deal with it constantly. Every time i hear a friend say it my comeback is, "do you know what you say when you say that's so gay" their responce is normally "i dont care" its really stupid. irritates me bc im gay. its about as bad as "that's retarded" i have seen the commercials about it. makes me feel better. although its still used wayyy too much. but thank you for not saying it. you're one of the few.