what are we supposed to say again when christians ask us where we get our morality?

[GrandizerII]

A large part of the reason that slavery was condoned was because of the belief in the inherent inferiority of those whom were enslaved.

Yeah, but this is not, in any way, due to observation of nature but due to a change in standard which is not based on natural observation. Nature says nothing about the status of beings.

Yep, nothing is standard-free, but standards in logic and mathematics and science are generally based on what can be observed naturally. The earth is round as that's what nature reveals to us. Nature says nothing about the right/wrong of slavery or theft or any other moral act/thought.