I don't reject that you can create an epistemically objective basis for a moral argument using the biological function. My moral arguments are always based on epistemically objective bases and that includes a variety of functionalities that I value across various fields including biology, psychology, sociology, politics and so on. You may argue that moral arguments based on biological functions enjoy increased efficiency and precision and I'm sure you'd have some good points to make about that. Relative to many other axioms that exist, I consider yours to be highly practical.However, my position is saying more than just that morality has a biological function, since from the biological function we can then construct an epistemically objective basis for moral judgments once we understand that function.
You say the purposefulness of human action is your axiom and not well-being and let me tell you, nobody is more concerned with purposefulness of action than me. I consider this to be a highly valuable way of looking at the issue of making distinctions on behaviour. Particularly because I believe values do not possess intrinsic value, my way of appraising them is by how well they carry out their intention function and then I appraise the overall worth of that value by whether the intended function was able to give the individual what they wanted. This process continues nearly forever, until you are doing everything for a reason that makes sense within the context of all the things you want.
You laughed at me before for saying capitalism has an influence on people's' behaviour but it's precisely things like capitalism that develop my interests that lie outside of biology. A big question for me has always been - to what degree society is ordered on the nature of humans versus the political, social, cultural and economic systems which we have created. What I mean is to what degree would something like capitalism have similar or identical impacts to an intelligent species other than humans as it does on humans? Causality and it's child purpose exists everywhere and in everything, people often seem enamoured and consumed by it - their design distorted by it. I like to look at capitalism as a separate entity from human civilization and study it's causality in order to make out what exactly it is.
This interest for me extends to morality and values, they too seem to have their own nature and causality which has evolved beyond what human's got from evolution. Moral distinctions exist today which were simply not possible before, values have changed substantially over time and the cause appears to be outside influences. Moral values travel as all infomation does, over boats, books, stories and now the internet. I could become consumed by entirely new principles after being exposed to an enlightening document on the internet which challenged my current thinking.
So I don't reject that biology is significant or that you can't use it as the basis for your arguments. It's just that morality, ethics and values appear to be more than just biological constructs. Hence holding opinions purely based on their biological function seems to me, itself a value. Perhaps not in nature but I know it's not the same as my perspective.
We may do things for the sake of reducing uneasiness but this is yet another biological function which I do not have to place any importance in. I am happy to have a debate about causality and efficiency regarding this but as far as objective morality goes - I don't see it. If you do not see functions as making distinctions between behaviour redundant then what's the basis for your objection to moral relativism? It seems to me you are confusing the best approach with the right approach - I believe I have the best approach too but I recognise it's my opinion, though I don't recognise it's merely my opinion.