"You know what might help? I'm 19 years old, so my group of female friends is in the 18-21 category." Not really. I'm 24, and sort of a grad student and I have friends that range from 19 - 30, almost all university students or recent graduates. I understand why, I even find it natural to use the word girl too. It's a conscious effort not to, and I was resistant to the idea for a long time myself.
The point is this. Our society is structured in such a way which is intended to make people automatically think less of women than they do of men. You might disagree with this, but I don't think of that as a controversial point. If you do have a problem with that statement, this conversation needs to go deeper than I realized.
Assuming you are on that page with me, I'll move forward. Part of the structure of society is bound up in the words we use, and the words we think are appropriate to use in certain settings. In its strict usage the word "girl" refers to a female child. It also is used colloquially to refer to mature, adult women. Do you not see how casually referring to adult women with the same term you'd use for a 5 year old is perhaps not the most respectful approach? And you might say "Oh but I don't mean it like that." or "Oh but they know not to take it that way." and I'm not doubting that either of those statements is true. But the fact is, we need language to think, and our language use really does impact the way we think about things. You might say "Oh that girl over there" and not actually be thinking of her as a 5 year old consciously, but your immediate unconscious reaction isn't quite so easily controllable.
'For instance, I would not address a group of female friends as "Hey women".' - Would you say "Hey girls"? That just falls on my ears so strangely.
Would you say "Hey ladies"? Would you then fix the jukebox by hitting it with your fist, and then saying "aaaaaaaaay!"
Why not just "Hey!"