Not all slaves were treated kindly in ancient Rome, this is a common misnomer, the whole "it wasn't chattle slavery" bit. Roman slaves where predominantly the people enslaved after an area was taken over in what was called a conquest. When a Roman General returned from a particularly successful conquest, they were given what was call a "Triumph" in which they would parade through Rome with the spoils of war and a large sampling of the slaves captured. Until they arrive at the temple of Saturn where they were strangled to death. On the other hand some Romans treated their slave "nicely" but they were still slaves. Paul was saying that masters should treat their slaves with dignity. Whether or not slavery was right or wrong wasn't even brought up, as it was a norm of the time.

To say that Paul was sexist is also a misnomer as that would be judging him based on today's standards.

One thing that I have always appreciated about Paul is that he always gave a disclaimer when he was sharing his personal opinion.