Why is gay used as a bad word?

Have you ever been talking to your friends and you hear them talk about a bad movie? Well i have and they never take the time to think cause automically when the movie topic comes on they say "that movie is so gay". It can be with any topic, "this birthday gift is so gay" or "this test is so gay" or "this music sounds gay". I know that it's integrated in society but why is it like that? Why can't it be changed? Where did it come from? I'm gay myself and i think it's very rude. Why can't they just use words like that's so stupid, that's so weird or that's whack? I just don't get it.