Translation
Does not the very nature of things teach you that if a man lets his hair grow long, it is a disgrace to him,
Paraphrase
Isn’t the general and instinctive understanding of your culture that if a man who grows his hair long it is a disgrace to him,
HOW DID NATURE TEACH SHORT HAIR FOR MEN?
The point being made here is that all the various cultures in that area of the world during that time in history were in agreement that generally, a man should have short hair, not long hair like a woman. Of course there were exceptions; in Israel those who took a Nazarite vow could not cut their hair as long as the vow was in effect, and those who wanted to show rebellion against authority grew their hair long, like Absalom.
DID JESUS HAVE LONG HAIR?
I think He did not have long hair. There is no evidence that Jesus was under the restrictions of a life-long Nazarite vow like Samson had been. We deduce this because it appears that he did not refrain from drinking wine, as was the custom in His culture, and He touched dead people (in order to make them alive again), which was strictly forbidden for those in a Nazarite vow.
As noted above, if a man grew his hair long, it was sign of rebellion against authority. This would be uncharacteristic of Jesus.
Where did the idea of a long-haired Jesus come from? According to the internet, it probably arose from people wanting to depict Jesus in the same way that they depicted Greek and Roman gods, especially Zeus, with long hair.
It is most likely that Jesus trimmed His hair like most other men of His culture at that time.