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.
Wait are you saying it's ok to say 'dumb' but not 'gay'? Because really that's just as insulting (to an even smaller minority).
As long as it's not obviously being used as a slur, I think using the word gay is fine.