As about half of the enlisted personnel I know would say, "Don't call me sir, I work for a living."

Translation: "I am not an officer, and therefore am not to be called sir. You can tell I am not an officer because I get my hands dirty and actually do things instead of order other people to do them for me"

It's mostly meant as a joke, but I could see how one might consider mistaking an enlistee for an officer to be an insult. coughnavycough