It doesn't matter whether or not inequality had a different flavor of cultural sophistication or acceptance attached to it at the time. It remains that the culture is not a good example of equality.
If you take context into account, inequalities in Athens were actually pretty decent compared to other places.
For one citizens were equals (~10% of the population, males only). Women, again for that time, could enjoy a relative freedom and had a place in society. Slaves were quite notoriously well treated in Athenian households, more like mandatory work than inhumane treatment, and a comparatively high proportion would eventually 'earn' their own freedom in Athens. In the wider picture, don't forget that slavery was that age's "industrial" drive, you could not end slavery and have the means to defend the city.
If you look at Athens within its own space and time, it really was a prime example of philosophical and societal enlightenment. It would, as you know, go on as a political model to influence the Roman Republic institutions (sadly a pale copy in practice) and later on the Empire's culture, down to our modern days quest for democracy passing by Revolutions. Judging "humanity" is relative, it can only be done relevantly to a context —that of the deed, and the idea.
I have a suspicion that much of our moral advancement as a species is essentially a function of our technological advancement. We're not more humane because we have better and more strongly held ideals, but because we're allowed by technology the space to practice them.
There's many things that are widely accepted today, that will be considered wrong in the future. Two examples would be eating meat and circumcision.
Later on, machines allowed one man to put out the work effort of several, and things progressively changed, but it took centuries, for hard reasons that I observe are, historically, more economic than philosophical. You could argue that domination or independence should not have been the end goal in civilizations history and I could agree, but then we're not talking about human beings anymore and it all becomes an abstract thought experiment rather irrelevant to our world.