Nothing is sacred. Nothing. Religion, race, sexual preference, whatever. It's all fair game. It's when people hate and hurt when it's a problem (could I sound like a bigger hippie right now?).
Chances are if Jay in fact read the letter and he is the bigot that the author says he is, he'd probably think, "Fucking queen.." and send it straight to his garbage. It wouldn't change his mind and it might give him great material (for a change).
If you don't like Jay's "gay-bashing" jokes, change the channel. If you hear the same insipid jokes on the street, don't listen and keep walking. Let the morons revel in their stupidity. It's not your job to enlighten idiots. Because they won't understand. You'd probably have better luck trying to convince a brick wall to have a three way with you and a blind puppy.
All I know is that it's hard to call Jay Leno, bigot or no, a comedian. Bugger ain't funny.