Have you ever had a serious job with female coworkers? I'm going to guess "no," since your entire post is nonsense.
First, you are referring to informal situations. But, this is (supposed to be) a professional setting, and the rules are much different in a professional setting. Generally, you want to refer to the femininity of women as little as possible in a professional setting, and when you do have to refer to their femininity, you must be extra formal.
Here is the rule I go by: never call a woman a girl in a situation where it would be inappropriate to call her a bitch (she better be a good friend). And, never call a black man a boy because it is almost as bad as "nigger."
If you don't understand the reasoning behind these rules then you should find a good friend to explain them to you very carefully.
Sounds like you worked with some pretty uptight people! I would suggest buying them copies of Spice World. And no, "Hacker News" is not a professional setting, it's certainly informal. "Women" is itself a loaded term to many feminists, who may prefer "womyn", "womun", or some other spelling.
In order for "girl" to be insulting, you must subscribe to the notion that there is something wrong with being young and female. Accepting that premise is to unwittingly agree with perpetuating a worldview in which the very notion of female-ness is disempowering. "Girl Power" actually challenges the establishment; suppression of any reference to femininity reinforces the older male status quo.
That so many people are programmed to think "girl" is insulting just shows how far society still needs to go to free itself from counterproductive paradigms.
> never call a woman a girl in a situation where it would be inappropriate to call her a bitch
Your implication of equivalence is loony, and of course more evidence for the informality of Hacker News. Your post also suggests to me a useful reason to use the terms "girls" and "boys" -- to smoke out people who get offended so easily or are stuck with such old paradigms that they clearly lack the flexibility needed to work in a start-up. Valuable co-founders aren't stuck with old thinking, are hard to offend and easy to joke around with.
First, you are referring to informal situations. But, this is (supposed to be) a professional setting, and the rules are much different in a professional setting. Generally, you want to refer to the femininity of women as little as possible in a professional setting, and when you do have to refer to their femininity, you must be extra formal.
Here is the rule I go by: never call a woman a girl in a situation where it would be inappropriate to call her a bitch (she better be a good friend). And, never call a black man a boy because it is almost as bad as "nigger."
If you don't understand the reasoning behind these rules then you should find a good friend to explain them to you very carefully.