It's like "PC" ("I hate the gays and/or blacks and/or jews and I'm annoyed I can't just be open about that without someone calling me a bigot"), "pro-life" ("I refuse to allow a woman to make decisions about her own body and insist on trying to legislate my extremist religious views"), and so on.
I always thought "PC" was a funny term given the people using it are the most likely to be trying to tell everyone else what can/can't be taught in schools, what people can/can't publish, what people can/can't read in their library, etc.