A hard question. But I will try to give a brief answer.

Morality is an aspect of social custom. Roughly, it is those customs that are enforced especially vigorously. But an important here is that while some customs are somewhat arbitrary and vary from place to place, other customs are much less arbitrary. It is these least arbitrary moral customs that we most commonly think of as universal morality applicable to and recognized by all humanity.

Here's an example: go anywhere in the world as a tourist, and (in full view of a lot of typical people who are minding their own business, maybe traveling, maybe buying or selling, maybe chatting) push somebody in front of a train, killing them. Just a random person. See how people around you react. Recommendation: do this as a thought experiment, not an actual experiment. I'll tell you right now how people around the world will react: they'll be horrified, and they'll try to detain you or incapacitate you, possibly kill you. They will have a word in their language for what you just did, which will translate very well to the English word "murder".

But why is this? Why aren't customs fully arbitrary? This puzzle, I think, is best understood if we think of society as a many-player game. That is, we apply the concepts of game theory to the problem. Custom is a Nash equilibrium. To follow custom is to act in accordance with your equilibrium strategy in this Nash equilibrium. Nash equilibria are not fully arbitrary - and this explains right away at least the general point that customs are not fully arbitrary.

While not arbitrary, Nash equilibria are not necessarily unique, particularly since different societies exist in different environmental conditions, and so different societies can have different sets of customs. However, the customs of all societies around the world, or at least all societies with very few exceptions, share common elements. People across the world will be appalled if you kill someone arbitrarily. People across the world will also be appalled (though probably not as much) if you steal from a vendor - and their concept of what it is to steal from a vendor will be very familiar to you. You're not in great danger of visiting a foreign country and accidentally committing what they consider to be shoplifting, unless you are very careless. I recommend that if something seems to be a free sample, you check with the vendor to make sure that it is indeed a free sample before helping yourself to it. As long as you are not a complete fool, you should be okay in foreign lands, because your internalized concepts of what it is to steal or to rob, what it is to murder, what it is to assault, very closely match those of the locals.

Some people think, "murder is wrong because it is illegal, and law is created by government, so what is wrong is defined by government." But I think that social customs are for the most part not created by government, and I think that the laws against murder, against robbery, and so on, follow the customary prohibitions, rather than creating them.

By the way, it's possible that I've mis-applied game theory, though I think that the concept of the Nash equilibrium is simple enough that a beginner like me should be able to understand it. My knowledge of it is spotty and I plan to remedy this over the next several months, so if I've made a mistake here hopefully I will not repeat it.

Yeah, no one can ever seem to explain what "objectively wrong" would even mean. That's because to call an action wrong is to imply that there is a negative value placed on that action, and for that to be the case you need a valuer. Someone has to do the valuing. Maybe a large group of people - or maybe everyone - values the action negatively, but that is still nothing more than a bunch of individuals engaging in subjective valuation. It may be universal subjective valuation, or maybe they think it's God's subjective valuation, but if so it seems better to spell that out plainly than to obscure it with the authoritative- and scientific-sounding modifier objective.

Pardon me. I deleted my comment before I noticed that someone had replied. (I didn't think replying to Constant was going to be beneficial. To be honest I didn't share your perception of interestingness of the conversation, even though I was a participant.)

What the hell does it mean for something to be objectively wrong?

Very little practically speaking. It is a somewhat related concept to subjectively objective. It doesn't make the value judgements any less subjective it is just that they happen to be built into the word definitions themselves. It doesn't make words like 'should' and 'wrong' any more useful when people with different values are arguing it just takes one of the meanings of 'should' as it is used practically and makes it explicit. I think the sophisticated name may be something related to moral cognitivism, probably with a 'realism' thrown in somewhere for good measure.

There is an objective sense in which an analogy is good or bad, related closely to the concept of reference class tennis. Having one technical similarity does not make an analogy an appropriate one and certainly does not prevent it from being misleading. This example of 'for example' is objectively 'bad'.