A plumb-line is what people use to make sure that a wall is straight or whether the wall is off. It is the standard to make sure the construction is correct.
In this application, it is the line that helps us know whether or not our “morality” is straight and not slanted or bias.
So you mean a standard for morality.
That's easy: well-being vs suffering.
In simple terms:
Good are those things that maximize well-being / minimize suffering
Bad are those things that minimize well-being / maximize suffering
If you disagree with this simplistic foundation, then I have no clue what you are talking about when you use the word "moral".
I agree totally in the case of rape although in some societies, if at war, raping those who you are fighting is encouraged and viewed as proper.
Yeah... those are called warcrimes. Warcrimes are immoral.
So I don't really get your use of the word "
although"
In their society, you will find that they agree that “it is morally acceptable”.
And if you would debate them on that point and have them actually present their reasoning, you will see that they wouldn't be able to present a valid argument for it.
In our society, we might agree that helping the needy is correct, yet you can find two people who say “I worked for my money and it is mine to do as I want and if I don’t want to help the needy, I am moral in my right to do so.
Every time you give such an "example", you seem to completely ignore the fact that no actual argument is presented.
I stated clearly right from the outset that both parties would have to argue their case, using rational reasoning and evidence.
You completely overlook this every time and instead just seem to be saying that it's all just a matter of mere opinion which doesn't need to be defended, for which no reasons must be given.
If the plumb-line that God sets is “love your neighbor as yourself”, an external plumb-line, then two people don’t have to argue because it is already set and we can judge accordingly,
That is just a variation of the golden rule and it is not original to christianity at all.
Every human society ever, with and without gods, have figured such out.
And once again, that idea itself is ALSO something that can be defended through moral reasoning. An actual case FOR it can be made, which does not at all require any appeals to any authorities - gods or otherwise.
On that subject, whenever you try to defend your moral case by appealing to an authority, you lose the argument by default. Because you essentially admit that you have no actual reason or defense for your moral judgement. Even if we agree on the general conclusion.
When the question is "WHY is x moral / immoral", the answer "because Y says so" is never valid.
Why is murder wrong? "
Because the government says so!"
Or to take one of your own examples: why is rape in war moral? "
because the warmonger commanders said so!"
Well, no....
When appeal to authority is seen as valid, that's when you end up with otherwise decent people engaging in attrocities.