There can sometimes be a herd mentality in male friend circles, that's all I got.

I expect that it was cultural in origin. In my experience as a school teacher boys in puberty tend to here together and become a law unto themselves. With patience and love they do come out of it. I think that when that are together they tend to act and react without thinking.

It came from community and civic leaders, like our dear old president, the king of the misogynistic douche bags, promoting an environment where toxic masculinity is rewarded and encouraged. Leadership matters.

I'm guessing he's fairly young, probably a teenager, and, if so, he's probably trying to "fit in" with other lads his age. Sadly, very few boys are lucky enough to have a parent like you who teaches them about respect, so a lot of those lads aren't going to know what he knows (they'll have learned all they know about interacting with females from TV, pop music and male adults who also didn't have a parent like you) - and I think he does know, deep down, that sexism and disrespectful behaviour towards women is wrong. Peer pressure and the desire to become part of the herd are immensely powerful things when you're a teenager. I don't think you should blame him. Just tell him why you feel disappointed and ask him how he'd feel if a man treated you or his sister like that; my guess is he's a good enough person that he'll feel awful about what he did and never do it again. Hopefully, he'll also resolve to stand up to it and tell other boys why sexism is wrong in future too.

I believe they do learn it at home (not yours), at school, at work in old movies. Often times it begins innocently with the boys will be boys excuse, macho crap, being told by peers, "girls like a take charge guy". Seeing bullies getting away with it, immaturity, insecurity... parents assuming, instead of teaching.

Peer influence perhaps?

Depends on how old he is and what he did. A lot of people attribute to malice things that can be much more easily attributed to ignorance or thoughtlessness.

Lots of possibilities, as has already been said here, but what is centrally important is this behavior being challenged. Whether by you or whomever else he will listen to he must, at the very least, take in the information that this is unacceptable. I hope it's not more difficult because he is an adult. And since he is your son don't forget the love part. I hope he will listen to someone. Good luck and peace.

Who knows as you can only guide them

Peers. And his president as a role model.

I'm not sure of the ages so it kind of comes from a lot of places..First guess is politics and politicians and wanna be orange autocrats. Then religion. Its all based on violence and asserting power. Other kids, who are probably the cruelest creatures. If they're older than school ages(even college) this is replaced by co-workers who never learned to be respectful. Could be a lack of discipline, or other family influences you aren't aware of. It could also be biological or neurological. younger kids do things that are cruel and confusing because of a lack of logical and empathetic developments. Even older folks might not have these fully developed and have issues dealing with emotional or mental issues. This is not a "boys will be boys" excuse, but rather that because those pathways aren't developed, both empathy and discipline as well as awareness and appropriate responses are needed. It might not have "come from" anywhere specific, but it is something that needs to be fixed.

Social structure. Kids are desperate to fit in with their peers, to appear strong and alpha. I did terrible things to people when trying to fit in. I bullied others, because I was desperate to feel part of a group. It was something I deeply regret, but it helped me understand pack mentality. "Those who show weakness will be left behind" in a group of people raised on "boys will be boys" and "suck it up and be a man. Grow some balls" even the most morally righteous can bend, even subconciously. The best thing you can do is remind him who he is and how he was raised. Explain to him how it hurts you to see this behavior. Make him know that he is better than that.

After about 10 or 11 years old, later for some, kids start emulating their peers more than their parents. My son had a moment like that when he was 11 or 12. His older (13 months older) sister informed me that I needed to kick his butt and why.

My son got religion. I got nothing to help you.

Society. Their peers? I'm sorry for the impact on you. Your story doesn't say how old they are so I don't know whether broaching the subject is a reasonable suggestion. But among other things, one possibility is that the story that got to you is not entirely or maybe not at all true.

My father was an arsehole. Cruel, sadistic, wife basher, child basher. My youngest brother and I both have kids, awesome kids and there has never been any violence in either home. So there is a lot more that comes into it than how they were raised. It is all complex. My father was spoilt, youngest of 7 and never hit as a kid, his own father was a true gentleman.

A combination of peer pressure, need to be accepted by them, and testerone

How old is he, if I may ask.

I'm a little confused ,You spoke of the boys you raised , then go on to say . "Their father" . Are you talking about the father of the person your son had the confrontation with ?

You left out the part just before he reacted. What provoked him? Maybe he was right.

Being faily old and have been a keen observer of humans, as my experiance and observations grew, the less I believed that personality was the result of nurture (upbringing/values instilled by parents, etc.), but that some/most people seemed to have been born (genetic pickup sticks), with their individual personalities. It would seem to explain the difference between siblings and a real rotten egg coming from a really good mother and father. It has also been my experiance that the more vigoursly a parent responds to a transgression the more it is likely to change future behavior...especially if the parent responds that way in only a rare occasion.

It comes from watching porn, which boys are doing younger and younger, and more and more often. About 80% of the mainstream porn available to boys is demeaning and degrading to women, and this distorts the boys' views of women, and sex in general. Find out how much porn he is watching, and get him some help. Video games and music videos are also notoriously mysogynist, influenced by porn.