I'm late to the party here but i think there's a huge problem with the way people perceive political correctness, even (especially?) in this thread.
There's language that is offensive. Racial slurs are plainly and obviously offensive. They are offensive because they are a deliberate insult and are intended to hurt.
Calling someone a chick or a "" or a "homo" or a "#######" is also very offensive. And despite the popular talking about how "special" has become offensive since it replaced "######", and autistic is starting to get used as a perjorative by kids today, it's still very much offensive to use any of those terms with a negative connotation. It's deliberately hurtful.
There's a simple litmus test for these words. If your niece had down's syndrome, would you make jokes that involved the word "#######" in front of your brother or sister? If not, then don't make those jokes in public, either, because you don't know the other people and any one of them might be facing a really difficult challenge that you're not up to. And if you wouldn't make those jokes in public, making them in private is kind of a jerk move, too. It's better to avoid it altogether.
But then there's words like "policeman" and "manhole cover" and "girls". For the most part, nobody is delivering these with the intent to offend. When somebody says "girls night out" or "hey ladies" they usually don't mean to imply anything negative.
But there really is meaning to those words. When you say a bunch of middle aged women are going to have a girl's night out, you kind of expect them to do some immature partying. Because they're not acting like adult women, they're acting like girls. Same thing goes with a bunch of men saying they're having a boy's night.
But if i had to pee and asked where's the little boy's room, it isn't offensive. If we had a couple's night and played a game and said let's play boys vs girls, it's not offensive.
When you refer to a coworker as a girl, you're not necessarily being offensive, but there's a good chance it carries an implication. If you refer to someone as the new girl in accounting, you might mean "the new accountant who is a female", but somebody else might hear it as every bit as offensive as if you said "that new black guy in accounting". Again, not exactly offensive, but it's really not cool. Because there's some negative implications.
But i think a lot of it goes back to the power of language. Replacing patriarchal language like policeman and fireman with police officer and firefighter makes it more socially acceptable for women to have those occupations. Replacing "girl" with "woman" levels the playing field be showing respect for an adult female. Those are good and noble goals for your own speech and also for the direction of the English language.
But here's the problem.
Lumping in words like "girl" and "policeman" with truly vulgar, racist, misogynist or otherwise offensive words as "politically correct" is bad for a couple reasons.
First, it encourages victim blaming. It's easy for people to say Why is everyone so easily offended these days? Stop being so sensitive.
Second, the broader the definition, the more difficult it is for people to change language they've learned since early in life. As the speaker, it's really frustrating to have to police your speech for fear of getting in trouble. And there's an implication too - that white people are all racists, that men are all misogynists, that we are all guilty of original sin, but some of us are guiltier. And that creates a backlash.
Which leads to the bigger issue. Others want to say the N word. Not as fair. Some people want to call their friend ###### when he does something dumb. That's offensive, but they might not see it that way. Some people want to be able to say policeman without getting in trouble. That's not on the same level.
When you call all of those things "politically correct", you provide cover to the racist, or the anti Semite, or the mysoginist, or the homophobe, and all the other hateful people who want sympathy for not being allowed to express their feelings. A perfectly reasonable person might complain that they can't say merry Christmas because it's not PC, and their racist buddy might say see, they won't let us say anything.
To me, this whole conversation about what's "offensive" is dangerous, not because it's important, but because we use the wrong language to describe it.
It's insensitive to say merry Christmas to someone who doesn't celebrate Christmas. It could even be offensive to someone who makes it clear they belong to a religion that doesn't celebrate the holiday. But it's not offensive to say merry Christmas in general. It's not "politically correct" to say happy hollidays.
It's insensitive to call women "girls" and men "boys". It might be offensive in a context where someone is deliberately diminishing someone else, like if a man refers to a coworker as a girl in a context where he's not referring to her gender, or if she replies that boys will be boys.
It's wrong to deliberately offend people.
We should strive to be sensitive, but people can be insensitive without meaning offense.
In most contexts, using the word "girl" is not offensive. In many contexts, it's insensitive, so you should try not to. There's also contexts where it's not insensitive or offensive, and taking unnecessary offense to it hurts more people than letting it slide.