Despite creeping up on my 31st birthday, I'm not really that mature of a man. Sure, I work a whole lot, pay my bills, I'm married with a kid, have responsibilities that I tend to, diet (when I feel like it), and take care of myself (for the most part), but I still feel like I never really grew up.
But if another man called me a "boy," I'd probably gouge their eyes out and pour vinegar in the open wounds. Or at least I'd want to because that's hella demeaning, and I think that there are lots of other men, who would agree with me (OK probably not about the violent response part, but you get the idea).
So why is it that so many people are quick to call grown women "girls"?