what are we supposed to say again when christians ask us where we get our morality?

[MindForgedManacle]

Ok, I am used to thinking of murder as killing that's considered unjustified, but let's go with the "legally wrong" definition.

Part of the reason I go with that definition is because the way you were going about it would seem to assume moral realism, which I don't think you intended. After all, you said that murder is wrong by definition, so I figured there were better ways to cash out that term.

In such a case, no, murder would not be objectively wrong in the moral sense. Plus, laws on what constitutes murder may differ with each state or nation, so even if a moral system supports the belief that murder, as defined by a certain state or nation, is immoral, it may not agree with the notion that murder, as legally defined by another state or nation, is immoral.

Again, I'm not sure what the point is here. Again, you're confusing ontology and epistemology, the implementation and belief with the actual facts of the matter.

If the law of a nation considers abortion to be murder, but another nation does not, which definition should the moral system of concern follow? Either way, it cannot agree with both notions of murder being wrong.

You're presuming that both are on equivalent factual foundations, which is patently absurd.

You are using a standard/ideal by which to measure the wrongness of stealing/theft. You are not relying on an observation of nature to determine that theft (in general) is wrong. Correct me if I'm wrong, but by what you're saying, you are basing the wrongness of theft on how you personally assess the well-being of each person involved in the theft and how you personally do the comparisons. If so, then how is this not subjective?

Are you trolling me? Of course I'm using a standard by which to measure the moral view of theft. That's how you mediate ALL "observations of nature". There is no standard-free observation or interpretation of anything for any being. But then you're again just begging the question by trying to assert a certain kind of evidence is required, when such evidence wouldn't even support the position in the first place.
And not quite. Consequentialists tend to analyze ethics in terms of the harm and/or benefits to sentient entities by their actions. Of course personal assessment is involved, but then it's involved in everything. But then to criticize that for using such would be to again make the epistemic-ontological confusion.

Adjustments in the fields of logic, mathematics, and science typically occur when new discoveries are made in nature (or in deduction and such).

For example, it was once believed that the earth was flat because that's what they could intuit at the time. But eventually, smart people came along, made some observations and calculations about the world they were in, and found that the earth was actually round (not flat). So a change in thinking regarding the earth's shape occurred as a result, not based on human standard, but based on what was observed in nature.

When did I say changes in thinking never occurred from observations of nature?

When it comes to morality, changes in thinking regarding the moral rightness of something typically occur when standards are adjusted rather not when a new discovery in nature (or a discovery of an argument based on axioms established by observation of nature or any such thing) occurs.

Slavery was, generally speaking, morally acceptable back in the days because standards were different from today's standards (in most of the developed world at least). But nothing in nature screams "slavery is wrong" in the same way that nature (from what is observed) screams "2 + 2 = 4" or "squares can't also be circles".

Again, a confusion between how we come to know what is moral and what is actually moral. A large part of the reason that slavery was condoned was because of the belief in the inherent inferiority of those whom were enslaved. That is in large part why the standards were different, because of what people believed (I suppose it's a sort of feedback loop).
Of course not. Those aren't the same sort of thing, because you're essentially just talking about contradictions with those.

You also keep mentioning the many branches in the fields of logic and mathematics and such, but that's still missing the point. You can't just say they're similar to many moral systems relied on just because they also have branches and different systems.

That's not what I'm doing. I'm saying they're similar because they have they work from axioms and inference rules to derive consistent conclusions, which is what is done in maths and logics. Not merely because there are branches and different systems, but because they have relevantly similar features in how they're developed.