You're interpreting "I'm morally in the wrong" to mean something like, "Other people will react badly to my actions," in which case I fully agree with you that it would affect my winning.

Actually I am not. I am interpreting "I'm morally wrong" to mean something like, "I made an error of arithmetic in an area where other people depend on me."

An error of arithmetic is an error of arithmetic regardless of whether any other people catch it, and regardless of whether any other people react badly to it. It is not, however, causally disconnected from their reaction, because, even though an error of arithmetic is what it is regardless of people's reaction to it, nevertheless people will probably react badly to it if you've made it in an area where other people depend on you. For example, if you made an error of arithmetic in taking a test, it is probably the case that the test-grader did not make the same error of arithmetic and so it is probably the case that he will react badly to your error. Nevertheless, your error of arithmetic is an error and is not merely getting-a-different-answer-from-the-grader. Even in the improbable case where you luck out and the test grader makes exactly the same error as you and so you get full marks, nevertheless, you did still make that error.

Even if everyone except you wakes up tomorrow and believes that 3+4=6, whereas you still remember that 3+4=7, nevertheless in many contexts you had better not switch to what the majority believe. For example, if you are designing something that will stand up, like a building or a bridge, you had better get your math right, you had better correctly add 3+4=7 in the course of designing the edifice if that sum is ever called on calculating whether the structure will stand up.

If humanity divides into two factions, one faction of which believes that 3+4=6 and the other of which believes that 3+4=7, then the latter faction, the one that adds correctly, will in all likelihood over time prevail on account of being right. This is true even if the latter group starts out in the minority. Just imagine what sort of tricks you could pull on people who believe that 3+4=6. Because of the truth of 3+4=7, eventually people who are aware of this truth will succeed and those who believe that 3+4=6 will fail, and over time the vast majority of society will once again come to accept that 3+4=7.

And similarly with morality.

Whether someone is judged right and wrong by others has consequences, but the people doing the judging might be wrong. 

Actually being in the right increases your probability of being judged to be in the right. Yes, the people doing the judging may be wrong, and that is why I made the statement probabilistic. This can be made blindingly obvious with an example. Go to a random country and start gunning down random people in the street. The people there will, with probability so close to 1 as makes no real difference, judge you to be in the wrong, because you of course will be in the wrong.

There is a reason why people's judgment is not far off from right. It's the same reason that people's ability to do basic arithmetic when it comes to money is not far off from right. Someone who fails to understand that $10 is twice $5 (or rather the equivalent in the local currency) is going to be robbed blind and his chances of reproduction are slim to none. Similarly, someone whose judgment of right and wrong is seriously defective is in serious trouble. If someone witnesses a criminal lunatic gun down random people in the street and then walks up to him and says, "nice day", he's a serious candidate for a Darwin Award. Correct recognition of evil is a basic life skill, and any human who does not have it will be cut out of the gene pool. And so, if you go to a random country and start killing people randomly, you will be neutralized by the locals quickly. That's a prediction. Moral thought has predictive power.

It is still an error to make morality justify itself in terms of instrumental utility, since there are plenty of examples of things that are instrumentally right but ethically wrong, like improved gas chambers.

The only reason anyone can get away with the mass murder that you allude to is that they have overwhelming power on their side. And even they did it in secret, as I recall learning, which suggests that powerful as they were, they were not so powerful that they felt safe murdering millions openly.

Morality is how a human society governs itself in which no single person or organized group has overwhelming power over the rest of society. It is the spontaneous self-regulation of humanity. Its scope is therefore delimited by the absence of a person or organization with overwhelming power. Even though just about every place on Earth has a state, since it is not a totalitarian state there are many areas of life in which the state does not interfere, and which are therefore effectively free of state influence. In these areas of life humanity spontaneously self-regulates, and the name of the system of spontaneous self-regulation is morality.

Yeah, I said it's not an argument. Yet again I can only ask, "So what?"

So being wrong and not caring you are in the wrong is not the same as being right.

(And this doesn't make me amoral in the sense of not having moral sentiments. If you tell me me it is wrong to kill a dog for no reason, I will agree because I will interpret that as, "We both would be disgusted at the prospect of killing a dog for no reason." But you seem to be saying there is something more.)

Yes. I am saying that moral sentiments can be wrong, and that that can be realised through reason, and that getting morality right matters more than anything.

The wordings "affect my winning" and "matter" mean the same thing to me.

But they don't mean the same thing. Morality matters more than anything else by definition. You don't prove anything by adopting an idiosyncratic private language.

I take "The world is round" seriously because it matters for my actions. I do not see how "I'm morally in the wrong"* matters for my actions. (Nor how "I'm pan-galactically in the wrong" matters. )

The question is whether mattering for your actions is morally justifiable.

The fact that you are not going to worry about morality, does not make morality a) false b) meaningless or c) subjective. Can I take it you are no longer arguing for any of claims a) b) or c) ?

I've never argued (a), I'm still arguing (actually just informing you) that the words "objective morality" are meaningless to me

You are not actually being all that informative, since there remains a distinct supsicion that when you say some X is meaningless-to-you, that is a proxy for I-don't-agree-with-it. I notice throughout these discussions that you never reference accepted dictiionary definitions as a basis for meaningfullness, but instead always offer some kind of idiosyncratic personal testimony.

and I'm still arguing (c) but only in the sense that it is equivalent to (b): in other words, I can only await some argument that morality is objective. (But first I'd need a definition!)

What is wrong with dictionary definitions?

You have not succeeded in showing that winning is the most important thing.

I'm using the word winning as a synonym for "getting what I want," and I understand the most important thing to mean "what I care about most."

That doesn't affect anything. You still have no proof for the revised version.

And I mean "want" and "care about" in a way that makes it tautological. Keep in mind I want other people to be happy

Other people out there in the non-existent Objective World?

, not suffer, etc. Nothing either of us have argued so far indicates we would necessarily have different moral sentiments about anything.

I don't think moral anti-realists are generally immoral people. I do think it is an intellectual mistake, whether or not you care about that.