Morals are based on societal standards, not some ingrained instinct. How can you argue true morality while societal bias inhabits everything the both of us debate?
The argument is that there's a thread, or a code, or a "Natural Law," which weaves through all societies across all of history - that what we see in any given group of people at any given point in time is a deviation from the thread, like the error from the trend line, uniquely caused by the circumstances of that group.
What you see as a societal bias, others like myself see as . . . well, bias, an effort to justify the particular failings of that group as acceptable under the Natural Law.