Using the word "gay" when you mean "stupid" is insulting. I don't care if you didn't mean for it to be insulting. If you wanted to say something was dumb say it was dumb, otherwise you're looking pretty dumb in my eyes.
Really, if you are at all close to someone who is gay you won't use the word except to mean, well, gay. You just won't. That one time you did use it in front of them it was really, really awkward and you realized it was the wrong thing to say even before your friend or relative even called you out on it. Because you're a considerate person, right? I mean, for you to use "gay" to mean "stupid" when you know someone who is gay makes you terribly insensitive.