Is gay a synonym for stupid?

I say phrases like "that's gay" all the time. Some people say its ignorant to do so but I don't really get why. I'm honestly not trying to be offensive to anyone. Is it wrong to use gay in that fasion? I mean doesn't language evolve constantly? Please don't give me links to dictionaries because honestly we all know people use gay to mean "stupid" whether Mr. Webster says so or not.