Well, i think you made a pretty good point. Maybe i really am rationalizing.
But about your example with another civilization, don't you think there's a pretty big difference between two civilized (even if at different levels) species, where the more advanced one could recognise the fact we are just at a lower grade but still "respect" the fact we are sentient, and between humans and animals?
I mean, even if they are on another stage of civilization i dont think we can subjogate like those tribes who lives uncontacted in islands or forests, cause they are still people.