On a good day:
"What to you mean by that? If you are intending to proclaim that I am happy, then yes, I currently am! If, however, you were attempting to state that I am homosexual, then I am afraid you have confused my demographic, for I am in fact bisexual! (Often I omit this part.) But if you intended the phrase "gay" as some form of insult, then I am afraid I have no time for you, as the belief that those of a non-heterosexual romantic inclination are somehow "inferior" is a load of preposterously moronic bullshit! All humans have the potential to be both exemplary and despicable, their demographics irrelevant! Now that I am finished with you, bigoted numbskull, you may go!"
On a bad day:
"FUCK OFF!"
EDIT: Sometimes I say "You're half-right Predacon!" I love quoting Tigatron in thoroughly inappropriate situations.