Reviewing some old posts, I stumbled across a comment from a professional philosopher (whose pseudonymous identity is no longer active, so he will remain anonymous) asked a question, which I will paraphrase from memory. Suppose some society did (some horrible thing) that everyone in that society subjectively approved of. Would I be all right with that?
The question though, makes a category error. The whole point of subjectivism is that the subjective is prior to a moral evaluation. And that goes as much for my own subjective opinion as for the subjective opinions of the members of that society. So, no, I would not be all right: the "(some horrible thing)" refers to something I would have a very negative opinion about, and my opinion is prior to and independent of the opinions of the members of that society. The fact that subjective opinions differ is of no more moment than that the acceleration due to gravity is different on different planets and moons.
The real question is, given that the members of some society approve of some horrible thing, and I disapprove of it (perhaps strongly), what should I do about that fact. Obviously, I have a motive to change the horrible thing; this motive is an identity, not an entailment: disapproval is a motive. But precisely what I actually do is dependent on a lot of other contingent factors, and can range from shutting up and minding my own damn business to making war on the people doing the horrible thing to force them to stop.
Subjectivism does not mean that if I subjectively disapprove of something, then that something is objectively wrong. That position is, I think, more accurately labeled as intuitionism, and is an epistemic theory. It's also so obviously contradictory, because people do have different subjective feelings about things, but an objective property of a thing must necessarily be singular, that only a philosopher could hold that position. Subjectivism, is an ontological theory: things simply do not have objective moral properties to be singular: moral properties are relational, not intrinsic. In much the same sense, the property of "to the left of" is relational, both of the object's relation to another object, and the speaker's relation to the objects.
Another way of looking at it is to consider people who disapprove of things that I (and hopefully you, gentle right-minded reader) approve of. For example, suppose Alice feels profound moral disgust at homosexuality, the exact same moral disgust that you and I might have for exploiting the proletariat or subordinating women and Black people. And, of course, you and I do not feel any moral disgust for others' homosexuality.
As a subjectivist, I cannot say that it is morally wrong that Alice finds homosexuality morally disgusting. Alice's subjective feelings are prior to moral evaluation, not derived from it. The question is not to morally evaluate Alice's feelings, but to figure out what we should do given a difference in our feelings. Given different underlying moral feelings, the question, I assert, moves from ethical philosophy to the realm of politics, in the sense that politics is what people do when they disagree about justice. We negotiate. We persuade. We can move the discussion to the meta-level: perhaps Alice would agree that her own unique sexuality should not be regulated by society, even if a majority of her fellow citizens were to find it disgusting, and then we would have subjective agreement at the meta-level. But these are political questions, not philosophical questions: we are not trying to get at the truth, because there's no truth to get at.