I think people just say it as it has come to a point where it is just used to call something stupid, although there are people who say it to be derogatory towards gay people. I don't think most people say it to be derogatory but you can never really know.

I think it's just a way for people to express their feelings at the time even though the meaning is "(of a person, especially a man) homosexual.". It really just depends on the context they use it in. If someone says "I can't do that." and another person says "That's gay" then it's just them being disappointed. However, if someone says "You are disgusting, you are gay." then its definitely to be derogatory.

Overall, I don't think it is that bad that people use gay in this way, I don't know how a gay male would feel as I'm straight.