I'm really not sure there's much if any intentional thought to offend when someone uses that phrase.

I'm not saying that means someone can't find themselves offended or hurt when hearing it.

I just don't see a person making it a point to take a stab at gay people when they say something like "That's gay!".

It's just became part of the modern day vernacular.

So, whereas I think someone's entitled to be offended when they hear someone exclaim "that's so gay!" because the negative roots of that phrase are obvious, I don't think it'd do much good to get too worked up over it. Just about every ethnicity and minority and group has dealt with it.

Ever welshed on a bet? Ever known an indian giver? Ever gone dutch on a date? Heard someone say "That's white of you"? Ever had an appliance that was "on the fritz"? Ever followed a swear with "pardon my French"?

Each of those phrases originated from one group of people having a negative perception of another group of people. As offensive as they are and were when they originated, the plain truth of the matter is they're such common parts of today's language that no one really thinks of the origins anymore. When someone says "pardon my French" no one really thinks of what the French have to do with that swear word or think to themselves "Ugh. Those foul-mouthed French!".

"That's so gay" will end up just like the rest of those phrases. Not very flattering, and still bound to offend some. But an integrated part of the vernacular that really has absolutely nothing to do whatsoever with gay people but instead holds its own meaning unto itself and neither demonstrates in the user nor inspires in those hearing it a negative thought about homosexuals.