I dont know about everyone else, but to me it seems that most of my friends think that being gay is the single biggest part of my personality. They constantly talk and make jokes about it and even introduce me by saying "This is ____, he's gay!" How do I get them to stop?
It's something different to them and it's natural for them to treat it like a big thing. I think that telling them your preference or reasoning with them will work best.
Get in their first and start saying "Of course you all know _____, she's straight, can you believe it?" The last part is vital. When they say "This is ____, he's gay!" the implied emphasis is "this is different, notice it". Since 'straight' isn't exactly 'different' you need a little extra. Adding 'can you believe it' adds that same emphasis, and at the same time points out just how insulting it is to have your orientation told to people like its something to be shocked about. Chances are they won't like it, and you have your leverage.
For some reason, I've noticed that girls (especially girls) tend to treat their "gay best friend" as if they're a pet, or something super novel to be envied-like a purse. It's weird. I have a LOT of gay guy friends and girls would always so how "jealous" they were…I didn't think of them as "gay friends" just "friends", and I would tell them that's probably why they didn't have any. I don't think they're doing it to be jerks, so just tell them nicely that it makes you uncomfortable and that you don't like being treated like a novelty, that you just want to be treated like a human being.
