@joefromchicago,
That's where you are wrong Joe, of course I am a moral relativist. Cultural relativity is the same idea as scientific relativity.
In scientific relativity people in different frames of reference will get different results to experiment. Each person, although they get a different answer, will be right within their own frame of reference. There is no "universal" right answer.
The principle of relativity from science is: Every Frame of Reference is equally valid.
In order to do physics, you need to recognize what frame of reference you are working in... otherwise you won't get the right answer for that frame of reference. But often when talking to other sentient beings from the same galaxy (let alone the same planet) the frame of reference is implied.
So when I say that liberty and equality are right, I am clearly referring to a Modern Western frame of reference. And since you are also in a Modern Western frame of reference, we can relate to each other just fine with this tacit understanding.
That doesn't invalidate the statement that other frames of reference (i.e. cultural contexts) are equally valid.