It's OK if women refer to each other as "girls", like Tracii mentions. It's not OK if men do though, in some instances (at work, or among strangers). Just like the N word. It's not OK for white folks to use it but African Americans do use it among themselves.
This goes to show how powerful language is, and the extent to which it influences how we feel or think about something. Imagine the mindset of a white person who uses the N word regularly when referring to African Americans vs a white person who makes a point not to (out of respect). Although less serious, it's the same with men who refer to women as "girls" as opposed to "women". Using "girls" can be condescending even though the person who uses the term doesn't realize he condescends. I explained why in my prior post.
I know it's difficult to be sensitive to all of this, but we really do need to try if we're all going to get along and pull together. Here's an analogy. You're tickling a young child who tells you to stop. Do you believe that the child has had enough and respect him enough to stop, or do you continue because you think he should enjoy being tickled.