Hi Makaze, sounds like you're confusing my position with some kind of consequentialism, maybe utilitarianism? To be clear, what I gave is a first principle. Obviously you're going to need things like Mill's harm principle on top of that, in order to get a world-apt ethics going.
Also, "But which one is more right? That inevitably comes down to a subjective decision." is very much begging the question. We can, do and must decide our actions according to criteria that allow us to justify ourselves to others. My argument is that we do so, by and large, already according to the principle of rational autonomy. Your engaging me in debate also presumes such a principle, for example. From there we can get to inter-subjective morality pretty easily.
None of this is to dismiss the complexity of ethical decision making - it is to point out the foundation upon which it is based - a foundation that is ignored in relativism.
I think I overacted to the word "easilly" in there. I've certainly spoken to people of a certain libertarian bent who take that principle and see it as the sum total of the discussion while ignoring the fairly obvious potential for contradictions and caveats. I certainly agree that it's a sound guiding principle, both from personal and societal standpoint. But since it's insufficient by itself and the truly contentious issues tend to lie in what additions are chosen to round it out the word easy struck a chord.