Just saying long hair is supposed to be feminine is a bit of a tautology. Particularly because acceptable levels of male hair length have changed over the decades.
I believe a larger factor is that long hair can look wild and uncivilized if it isn’t maintained properly. Hippies used to wear their hair long as an expression of freedom and rebellion.
And do we really want to return to the horror of the “man bun”?
As long as we can avoid the horror of the “white-collar mullet”: a generally short male haircut with a very thin 4-8" pigtail dangling off the back.
Everything has changed over the decades. The generalized point is that fashions associated with women are seen generally seen as degrading or absurd when men wear them. The reverse isn’t true, because patriarchy.
Long hair on women is not just a current feminine style, it is also a male sexual fetish. Historically, unmarried women might be allowed to display long hair but as soon as they married, it was supposed to be hidden in some fashion – available only to her husband’s eye.
As an easy guide to male fetishes, just notice what most women choose to wear when they age out of sexual desirability. Flat shoes, loose clothes, short hair. Much like what men wear. Because being a walking sexual fetish is usually physically uncomfortable. So long hair is also in that special category. Wearing the sexual fetish fashions for women, if you are a man, is transgressional. Seems to me that the classes of men most likely to sport long hair are looking to violate norms in some way or other.
And as I’ve pointed out repeatedly, long hair on men is not always considered “feminine”, historically or present day. But it is usually considered wild and/or unprofessional.
When they perceive that they’ve aged out, perhaps. As I’ve gotten older, I’ve noticed my appreciation of women my age has kept pace, and I expect that will continue.
I say that not to sexualize the thread, but to point out that women do NOT have an expiration date. (Not that you were suggesting that.)
Yeah that’s what I meant. For some this is a sorrow to them, to others, a relief. And of course for some, it never happens.
The men who wear long hair in my part of the woods are usually aging hippies who got in the habit. Or college students trying it on. It is a complex cultural phenomenon really. Like other appearance options, it conveys a package of messages which others pick up in a gestalt.
It general bespeaks a lack of interest in social norms – not a lot of insurance agents, CPAs, and bank managers sporting the look.
Nearly everyone is wearing a hat. Geo. wore a wig.
But yeah in the sailing age, sailors did wear long hair in a queue .
Which is very late and by that time health issues mean the wearing of high heels, etc is prohibited. My wife is about my age, and wears her hair as long as her health permits (it is now somewhat fragile and thin)
And there is a difference from 'sexually attractive" and a “fetish”.
a form of sexual desire in which gratification is strongly linked to a particular object or activity or a part of the body other than the sexual organs.
I was referring to the observably common fact that men tend to see specific body parts treated specific ways as erotic, and even separable from the person they belong to, in a way women really don’t, as a rule.
Fetish has a specific and a generalized meaning and I was going for the more general.
But this is a tangent and I don’t think it should continue in this thread. I’m also not wanting to get into another go round of a lot of unconsciously defensive men yelling at me, so I’m going to bow out for that reason also.
I’d there really that much dislike these days? These things tend to be about control or perceived gender norms, maybe a degree of animus against dreadlocks or turbans.
That said, although my opinion of pony-tailed men is neutral, as a group they sometimes have issues. 
No doubt these issues are more common in the military, wannabe military, controlling schools, socially conservative places, places that dislike and discourage individualism, etc. I wasn’t around in the 60s and could not guess how much intolerance was based on cleanliness, jealousy or transference of other social issues.
Obviously the bias is cultural, changes over time, and is primarily a matter of conformity. Secondary concepts like long hair being a feminine quality that men should avoid follow the conformity model.
Do any of you know someone who started avoiding barbers during covid and then decided they liked it? This happened with one of my sons. There’s been no blowback where he works. A few co-workers call him the IT barbarian.
I know a few.
It’s been nearly ten years since my last haircut. I always liked having long hair and had to keep it short for jobs over the years
I’ve had it short since i was in the USAF in the '80s, and when I got out I have kept it short. Get my haircut once a month.
FWIW, I thought your comment was perceptive, insightful, and intelligent.
That is fortunately not as bad as it was in the past. When my ex wife was in high school, she had to bring a note from her mother because her hair was not jet black. There were schools which made students dye their hair black if it wasn’t dark enough, and they also made students have their hair straightened. Some Asians have a little wave to their hair.
I think most high school girls dye their hair some sort of color, often a lighter black or dark brown, although some schools have no restrictions at all.
Some Taiwan schools have similar restrictions.
/hijack
I’ve grown my hair long since my late twenties. I’ve worked in public and school libraries, and have never had anyone complain about it. I did have a barber take it upon himself to cut too much of it off (I’d gone to that shop many times before without issue). My son had the same issue at a fancy place where the stylist decided he needed more cut off than he asked for. His girlfriend announced that she would take over cutting his hair. If my hair had thinned I would have stopped growing it long, but I’m approaching 60 and have a luxurious mane.
My story starts the same but takes an opposite tack.
I was in the Air Force for 21 years. For those 21 years I had to keep my hair short.
Now I don’t, so I will not cut my hair short until I have not had to for 21 years.
^^Congrats.
I like long hair on men. My husband has long silver wavy hair and looks like a wizard. 
Occasionally he’ll get it cut and look like George Washington. 