Meta-ethical subjective relativism is predicated on the lack of an objective (non-mind-related) ontological account of ethics that accounts for perceptual facts, in the same way that a singular, objective physical reality accounts for perceptual facts. We thus lack an evidentiary foundation for ethics. Furthermore, because of Hume's fact/value distinction, we cannot use physical reality as an axiomatic foundation for ethics.
It might be the case that there is some objective ontological account of ethics; we know it's logically possible because we do have a good physical ontology of perceptual fact. However, no such account has ever been rationally justified; all such accounts run afoul sooner or later of the perceptual facts. Notably, the content of ethical intuitions disagree, and when there is absolutely universal agreement, there is no need of further ethical discourse. The whole philosophical project of ethics is to resolve disagreements of ethical intuition; this project is dissimilar to scientific project, which is not about resolving disagreements of perceptual experience—science accepts as foundational only those perceptual experiences which are agreed upon universally prior to scientific investigation.
What we can agree on, in a shared-perceptual and scientific sense, is the existence of ethical intuitions as subjective entities, the existence of values, desires, and goals, irrespective of the objective truth of their content. Our minds demonstrably do not do nothing but draw objectively true conclusions about the physical world. We want things, we desire things, we like and dislike things, we have preferences. These preferences are important in and of themselves. Doing something just because one wants to do it is—everything else being equal—a good reason to do it.
Of course, everything else isn't always equal, and one's immediate desires are not the only reasons to do or not do something.
First, our minds are not unitary; they are multi-voiced. One part of our mind can want something, another part can want the opposite: We can be both attracted to and repelled by the exact same thing. Furthermore, our minds operate distinctly on different levels of abstraction: We can desire, for instance, good health, and can also desire specific activities which harm our health. Although the underlying objective states of affairs are rationally related, our ethical beliefs themselves are often not directly rationally related. I don't derive my preferences about ice cream from my preferences about my waistline, even though the physical effects on my waistline do derive from the physical nature of ice cream.
Second, each of us lives among others who also have wants, desires, values and preferences. Many of those preferences require limited physical resources to satisfy: The desire to be satiated requires the eating of food, and there is only so much food lying around. So the fulfillment of desire can cause conflict.
Third, given our particular biological and cultural evolution, we have wants, desires, etc. about other people's minds: We want others to want our company, to approve of us, to like us, to love us. Some of us also want others to suffer, to have their desires frustrated, to feel pain.
We can create quite a complicated discourse without ever saying a single thing about the ontological nature of the content of these desires, without ever objectively breaking the symmetry between physically or socially competing desires. We can't objectively break the symmetry. Since we can't do so, a desire to actually do so is doomed to frustration; the best we can do to satisfy the desire to have objectively true ethical beliefs is to descend into delusion about physical reality.
So what can we do? Specifically, what are we physically able to do?
First, we can propagandize: We can attempt to directly influence others' desires. Propaganda does not entail lying, though. Martin Luther King offers us one of the greatest and most uplifting pieces of propaganda. Watch his speech carefully. I don't know all the reasons why he's persuasive, but I know he doesn't make anything like an argument: Not a scientific argument, not a philosophical argument, not a mathematical argument. Just exhortations, assertions, and a couple of veiled threats. You simply cannot account for the persuasiveness of his speech or the 1960s Civil Rights Movement from a position of philosophical rigor or objective truth-seeking. Racial equality is not truthful in the sense that mathematical or scientific equality is truthful. It just—to many but not all—feels good... especially spoken with King's magnificent oratory.
We can negotiate. If you and I have competing desires, maybe we can share. Maybe we can take turns. Maybe you can give me this and I'll give you something you want. We can pressure. We can praise. We can blame.
We can even coerce. We can lock our front doors, each forcing everyone else to stay out of her house. We can strike back if struck. And, if we are are so moved, we can and do simply imprison or kill those whose desires arouse our greatest antipathy.
If everyone's desires were absolutely different, if there were no commonality, we would have chaos or pure individualistic competition, the law of the jungle. But we don't have chaos, therefore it follows that we actually do have commonality. Not should have, but do in fact have. And thus we have societies. But don't be fooled: The best scientific explanation for this commonality is that they are a product of the accidents of our unique biological and cultural evolution. We might have ended up completely different, and then we would be constructing our social ethics on a different subjective factual basis.
How can we best propagandize, negotiate, pressure and coerce? First of all, in the most abstract sense the question makes no sense without actual people with minds and existing values. We can't even talk about "best" in a neutral, objective sense: We must always construct "best" in the context of one or more existing mind. Secondly, even given some particular collection of people and their values, there is no "optimization" because each person has his or her own values and preferences: There is no single standard against which to call a solution optimal. Even a utilitarian, statistical evaluation entails making subjective value distinctions: Should we optimize total wealth? Some particular pattern of distribution? Which pattern? The only "optimal" solution would be for everyone to be perfectly happy, which does not seem achievable even in principle, much less in practice.
The best we can do is what we're doing: Each person propagandizes, negotiates, pressures, praises, blames, and coerces according to the dictates of her own conscience and her best rational evaluation of the consequences of her actions, and society is what emerges from every person doing so.