In my opinion, I can see how it would be offensive.
For one, the use of the word. People often use it to mean "stupid" or "dumb". That in and of itself is offensive. Kind of like, if someone were to call someone "black", it depends on the way they use it. If they say like, "Well, you know I don't like them because they're black". It's not the word itself that is offensive, it's the way they use it! Does that make sense?
So, when people say "That's so gay" I can understand why some people would say it is offensive. Equating dumb and stupid with being gay is wrong.