What are some reasons why it is a red flag for men to refer to grown, adult women as "girls"?

It indicates immaturity - it shows us that men are still mentally a teenager, using terms "boys" "guys" and "girls". Many men reminsce the good old school days because it was back when they could be freely immature and devoid of adult responsibility.

It also suggests pornsickness - the term "woman" on porn sites is practically absent. Women are either refered to as girls or mums, because it fetishises the idea that woman are subordinate to men.

It reeks of pedophilia/hebephilia.

I've witnessed men in their 40s, 50s, and 60s referring to the women they are dating, fucking, or aiming to fuck as "girls".

They are saying it's appropriate for grandpas to fuck girls under 18.

The disrespect to these women is a slap in the face.

It could also mean that he's stupid. At some point in adulthood you need to just realize that "girl" refers to children, and you realize that you and your peers are no longer children and so you get used to saying "woman" instead. You also realize, if youre not a complete dumbass, that's it's insulting to call an adult using child words. And, if you're not an absolute dunce, you realize that it's unethical to demean women, in particular, in that way.

So if a man calls women "girls", he's probably a utter imbecile with a weak grasp of the English language, low capacity for abstract thought, and passive contempt for women.

It's infantilizing.

It always made me uneasy it’s just something rancid about calling a grown women girl I had that to me the guy I was chatting with he would always start with this line “ hey pretty girl ” I used to be so confused and felt so uneasy like why 😵‍💫

Now that I know about pdffile culture in patriarchy it makes me think they are talking about actual girls. Think about it: If all men, normal and pdf, refer to women as girls, then how do we know who is an actual pdffile? It's the perfect cover really, they can talk about 🍇 in public with no pushback.

It could indicate that he's gay... a lot of gay guys seem to refer to women as "girls".