It's in an in-group vs. out-group thing. If a group decides something's offensive... then it is. You can't call a woman a bitch, but that didn't stop feminists from creating http://bitchmagazine.org/ . Gays are doing the same thing now. If you say "what are you a queer" you're being an asshole, but if you refer to "the Queer community" with a capital Q, nobody will notice. You can make rape jokes all you want, but say one to a rape victim. You can say cancer jokes all you want, but say one to a cancer victim.
Here's an example for gamers:
You can say to one of your friends who plays a lot of online FPS games, "After I own your ass I'm gonna teabag the fuck out of you." They'll most likely assume that you're using some weird metaphor about "pwning" them and then repeatedly crouching your character over theirs.
Then say it to somebody you don't know.
