You gotta stop looking at it from a literal point of view, and view it in the definition people are using it as. Whether you view it as being offensive or not is not going to stop people from using the word. Seeing your original post, you know what definition people use the word for. Gay and many other words mentioned in this thread have evolved and taken on new meanings which aren't the actual meaning, this is called slang.
I personally don't get offended by the word's common use, because I know whoever is saying it is not meaning any offense towards me or anyone else. It is rare for someone to use the word to actually describe something as being homosexual, and I have never heard the word being used to describe something as being merry or happy other than in old novels.
I know it may hurt your feelings when people use the word, I remember I got mad at someone one time for using it to describe something as stupid, but I realized they did not mean any literal offense towards anyone. Some people subconsciously use the word, as in it has just developed as a part of their vocabulary with them not actually realizing what they are saying. Do I take offense and hate them? No, because I know they don't mean it offensively, the word is used so much nowadays that I have just learned to ignore it, and replace it with the word they meant.
Besides, you don't have to go by gay. Gay has no definite meaning anymore in today's society, homosexual does though. If you look up the word homosexual in the dictionary, or if you ask someone the definition for homosexual, the only definition you will find is "Sexually attracted to those of your own sex." You will receive a lot of varying responses if you ask for the definition of gay. I don't recall anyone using "that is so homosexual" because frankly it doesn't make sense.