Hi, man here. I obviously can’t speak for women, but I can answer your question from my perspective.
“Girl” is a term used for young, inexperienced, non-adult female humans. When you call an adult woman a girl, you imply that she, her thoughts, and opinions are juvenile, and therefore are not to be taken as seriously as those of an adult of equal standing among other adults. It’s called “infantilizing” and is disrespectful to that women, and realistically to all women.
For a moment, imagine you’re with another man you don’t know and when you say something, he contradicts you and begins to address you as “boy” or “son” in his response. His intent is to diminish you as a person and make you feel small, inferior or less a man or expert as him.
Now, I don’t know what country you come from or if English is your first language; this cultural aspect can apply to other English speaking countries, but specifically I’m talking about the US. One could certainly be forgiven for speaking English as a second language or not being from the US and not knowing this specific thing. I hope this helps.
One last edit, or this is going to bother me:
Really… Being called girl is offensive now 🙄…
Responding like this makes me think you’re just an asshole and not interested in the actual reason, so perhaps I’ve wasted my time with you. That’s ok, because maybe there are other people out there around who can read this and then correct you in public for this shitty behavior
Oh, you ARE an asshole on purpose. This all makes much more sense