because being called a fag is the ultimate insult. for you lesser people here on the board it instantly puts you on the defensive, trying to defend your manhood, and sexual preference. once you drop the fag bomb, all other arguments are invalid, because your manhood has just been put into question.
joke time:
what do you call a gay guy in a wheelchair?
rollaids.
what do you call a gay guy in a sleeping bag?
fruit rollup