Jesus should not be portrayed with long hair because:
Nazirite Vow: In the Book of Numbers (6:1-21), the Israelites could take a Nazirite vow, a special vow of dedication to God that included several stipulations, one of which was not to cut their hair during the period of the vow. Samson, a famous figure from the Book of Judges, was a Nazirite from birth and had long hair as a sign of his vow. So it could be said that they normally would have shorter hair.
Priests: According to the Book of Ezekiel (44:20), priests were not to shave their heads or let their hair grow long but were to keep the hair of their heads trimmed.
Men and Women: The Apostle Paul, in his first letter to the Corinthians (11:14-15), makes a distinction between men and women's hair, stating that it is a disgrace for a man to have long hair, but a woman's long hair is her glory. However, this is a New Testament passage and reflects cultural norms of the 1st century AD, which may have been different from earlier periods.
So while it was not a command it seems, IMO when a man looks like a woman from behind with their long hair then it is an issue and one can often be mistaken as being a woman from behind as many of us have likely experienced.