I feel like this attitude develops from school. We learn about these heroes of the past, who bravely did things that people didn't respect at the time, but now we look back at them for being on the right side of the issue at the time.
There is a lot of merit in this, and I some times find myself thinking in the same way, but should I? What guarantee to we have that people in the future will actually have the right attitudes about things? Surely we've all seen science fiction stories in which future societies have very horrible cultures and attitudes about things. Are we just looking to be on the "right side of history" regardless of what that view is? Or do we somehow have this view that the future always ends up picking the right view on things? And if so, why in the world are we so sure of that?
You can use history as an example. Look at the dark ages in Europe. You could surely find things in which you feel the earlier views/practices of the Greeks or Romans were superior to the Medieval dark ages. Or look at the changing cultures in the Middle East over the past millennia. You can easily find time periods where we would look more favorably at was the views/practices in the past over what developed later on in some of the same societies.
So it makes me wonder, why are we so sure that history is a good arbitrator of what's right or good?