Then that wouldn't be what I was talking about
True, agreement does not make a thing moral, after all, morals arise from shared values, not agreement.
But just as with a system of measurement, once we agree on a system there are things we can say are right and wrong with respect to that system.
Step back and ask yourself, what is the purpose of a system of measurement? Whatever your answer will most likely be part or all of what we value in a system of measurement.
Take the imperial and metric systems. Why was it necessary to create a metric system if the imperial system was already in place? Because over time we understood, via knowledge and understanding that there was an even better way to achieve the goals of the system.
All I'm saying is that, like a system of measurement, morality is a system that exists relative to shared goals that arise because of the values we share. With respect to those goals, there are objectively right ways and wrong ways to act and behave.
That's it.
I'm not attempting to assert that values and goals set are necessarily the best. Best relative to what? I'm not saying that the objective nature of any act or behavior is objective in an analytic sense. If we learn something new, like; no human is inferior in a biological sense to any other human, this knowledge can and often does change our values and in turn our morals. The statement that was objectively true before we learned that can change because the objective nature of propositions always relate back to and are grounded in synthetic subjective propositions.
Take for following statements:
It is bad to experience both actual and potential unwanted suffering, pain, and sickness
It is good to strive to achieve both actual and potential desired health, happiness and well-being.
Fundamentally, are the statements above objectively true? Of course not, people chose to value these things because those statements are completely consistent with our experience. The universe doesn't care if we live or die, but we do. We understand what death is, at least in this reality. We know what pain is and the human body seems to have evolved (if imperfectly) to try to avoid pain and suffering) and we know our actions have consequences and (most of us) understand how those actions affect others (empathy).
With these things in place, there are certain actions we value and others we do not, and our morality arises as a result of our values.