Everyone seems to be missing an important point here: the Letters to the Corinthians were letters to Corinthians.
They weren’t letters to the Galileans or the people of Jerusalem. They were intended solely to be read and understood by the people of Corinth. In fact they were only intended to be read an understood by the small Christian congregation in Corinth
As such, all the the passage in questions tells us is that Paul expected the *Christians of Corinth *to agree that men wear their hair short. Nothing more. He isn’t even saying that men *should *wear their hair short. He is just saying that his readers, the Christians of Corinth, expect men to have short hair. That expectation is then used to illustrate an entirely different point about women wearing headscarves.
I don’t know enough of the history of Corinth or the Corinthian congregation there to say for sure, but it seems quite plausible that they were mostly Roman ex-pats or heavily Romanised. As such they would indeed have agreed that men wear their hair short, and that would be a natural way of illustrating Paul’s point.
But that would tell us nothing about the attitude of people generally about long hair on men. Even if every single Judean Christian wore hair down to their arse, the passage would make perfect sense so long as that was only a provincial Judean thing, not something done by the civilised, Roman folk of Corinth.
Even if all the other Apostles wore their hair long, the passage would still make perfect sense to the intended audience: the people of Corinth. It wouldn’t be seen as insulting to the other Apostles because it was never intended for them. It was intended for the people of Corinth.
Even if they found out about it, they wouldn’t be insulted because it’s an illustration. It is like me illustrating a point in a debate on this board by saying “You agree that it’s OK to use insecticides on our crops, therefore hunting…”. That won’t offend my friends who are organic farmers because it* isn’t meant to apply to them*. I am illustrating a point by selecting something that I know beforehand my audience will agree with. I’m not saying that it is obligatory to use pesticides. I am simply stating that, since my audience accepts Y, that same reasoning means they should accept Z. Since my audience accepts the use of poisons to kill pests, they should also accept the use of bullets for the same reason. If my audience accepts that men should have short hair and women long hair, they should accept headscarves for the same reason.
Even if Paul knew that Jesus himself wore his hair long, it would still make perfect sense, so long as the people of Corinth accepted that long hair was unnatural for men.
Paul was writing for a specific audience. His choice of illustrations can’t be used as a guide to the views of the Roman empire as a whole. It can only tell us about the prevailing views of the Christians living in Corinth. Clearly they accepted the Roman standard that men should have short hair. But that isn’t evidence that any other Christians accepted that standard.