The men of the Merovingian Frankish dynasty saw long hair as being somewhat symbolic of their power and pride, and it was dynastic tradition to keep it long as a result. The ruling dynasty would all keep it long and grow it out. They would grow it out long, and grow their beard out a bit too, never cutting it. Cutting that hair was a great shame, and one Merovingian forcefully cutting another's hair was a great shame and insult, and effectively made the one whose hair was cut unfit to rule. When infighting with one another over power, if one was captured his hair would be cut as a way of getting out the message that he no longer carries power or prestige. When a Merovingian was deposed by another their hair would be cut or they would be scalped to send a message.
When Pepin the Short, future King of the Franks and father of Charlemagne, deposed Childeric III he cut Childeric's hair before sending him into exile in a monastery. This would be interpreted as Pepin declaring that Childeric no longer had power. This cultural symbol seemed to pass away with the Merovingian dynasty after Childeric's deposition, as the Carolingian's were known for keeping their hair short.
I am uncertain if this was just a cultural tradition of the ruling elite of the Merovingian times in Francia, or if it was something that took place among all free men of the time. Perhaps someone with a greater knowledge than me can enlighten us on that fact. The importance of hair in the Frankish lands definitely has a mention in the Burgundian code, which predates this time a little bit. In the text it is listed that pulling hair is a crime, punishable by a wergeld. A wergeld was effectively a form of compensation paid to the damaged party, as a means of enforcing a legal system. This system of a 'wergeld' typically covered factors such as the killing of slaves, or family members, the stealing of property. You kill someone's kin, kith, or slave, you pay them, basically. It was a means to stop different families or clans killing one another over generations long blood feuds. A means of bringing some order about. However, the code also covered the treatment of another Burgundian's hair, and this was punishable with the payment of a wergeld. Whether they wore this hair long is unclear in the writing, but it seems implied that it was long hair.
If anyone siezes a native free man by the hair, if with one hand let him pay two solidi, if with both, four solidi; moreover let the fine be set at six solidi."
[The Burgundian Code: Book of Constitutions or Law of Gundobad- Additional Enactments (The Middle Ages Series), page 26]
It was also a crime to give another free man or slave false hair, such as a wig.
Whoever unintentionally provides a native free man or slave who is a fugitive with false hair (capillum fecerit), let him forfeit five solidi; if he provides him with such hair intentionally, let him be compelled to pay the wergeld of the fleeing man.
[The Burgundian Code: Book of Constitutions or Law of Gundobad- Additional Enactments (The Middle Ages Series), page 27]
As you can see here, the wording is slightly unclear on whether this is long hair or not. You can pull on short hair. However, it seems to be implied that they mean long hair. However, I don't really want to speculate on it.
If anyone is interested in a lecture on the Merovingians, where the importance of their hair is discussed, you can check out a nice Open Yale course by Paul Freedman, where he discusses at length the Merovingian dynasty, as well as their long hair pride.
[Part 1 of the Merovingian discussion.] (https://www.youtube.com/watch?v=3uMZVj5b0Qo)
[Part 2 - General Frankish society, where the Burgundian code is discussed] (https://www.youtube.com/watch?v=kNLLrgsAxds)
[Part 3 - Short discussion of Pepin cutting Childeric III's hair] (https://www.youtube.com/watch?v=diwsP8eOpvE)
What this shows is that at one time long hair for men was indeed something that was accepted and considered a traditional norm within the ruling class in a culture. I hope that goes part way to answering some of your question.
Sorry if those links or the formatting on this don't come out right. I have never linked anything on Reddit before, and rarely ever post.
edit: switched out 'fine' for 'wergeld' and added short explanation of what a wergeld is. A fine implies the money was paid to the state, which is untrue at best, outright inaccurate and misleading at worst.
This isn't exactly a culture-wide phenomenon, but it may be of interest.
At the very end of the 18th century, not too long before Napoleon's coup, a fashion arose for very short hair on women. It's often considered to have been a grisly tribute to the guillotine or its victims, whose hair would be cut in order to bare the neck, although there is no positive evidence of this as far as I'm aware. (It seems likely, but I like to be clear.) The style was called the "cheveux or coiffure à la Titus" in French, meaning "hair/haircut in the style of Titus", and was referred to as "cropt hair" or a "cropt head" in English.
The specifics of the cut varied - you can see paintings that show women who look like they've had a late 1920s-style shingle cut, often with some length left in front for curls, some with a bit of shagginess all over, and there's even a "crested" variation I can't find the illustration of right now that's basically combed into a mohawk - but since some men were wearing their hair a bit Romantically grown out, some of these women's hairstyles were shorter than some men's.
It's hard to date exactly when this trend stopped, or how widespread it was. The French Journal des Dames et des Modes would go back and forth, saying one week that the Titus was too generally worn, even by servants and shopgirls, and now anyone of merit was using false hair to hide the style, but then a few weeks later would show a fashion plate with a Titus and praise it. Jane Austen complained about her niece Anna's "sad cropt head" in 1809, which gives the style a later end date than you might suspect (given its general association with the Terror) and provides evidence that it wasn't solely a fad among the fashionable and shocking.