Theistic morality

[Purple Rabbit]

Historically speaking, there has certainly never been one underlying moral code which all people shared. That doesn't mean that there isn't a rationally based moral code which would have been true in the past, regardless of what people believed.

This is a contradiction. If historically there never was a shared common moral code than there can be none in retrospection. To posit one in hindsight would be a revisionistic action trying to rewrite history. You cannot just posit that moral positions on gender equality and slavery are absolute throughout time.

Denying this would pave the way for moral subjectivism or moral nihilism, and I think we've agreed that this isn't a practical viewpoint to hold.

If moral subjectivism is a good description of reality I don't see why we should avoid it. We haven't agreed that moral subjectivism is not a practical viewpoint. It gives an historically and culturally accurate description of reality. By asserting that the moral code for humans SHOULD be absolute and therefore IS absolute, you derive an IS from an OUGHT. Moral subjectivism states that there is no objective fact of the matter over whether a specific action is right or wrong. It does not exclude a moral code shared by the majority of world population.

You don't need any absolute moral standard for that. It's just an economic choice for synergy over battle that both involved parties can benefit from given the boundary condition are in place (like mutual trust of a certain degree). Also observe that the economic rule itself is an IS not an OUGHT. Given the right conditions it will play out no matter whether there is explicit rephrasing of it into prescriptive moral codes. You cannot derive an ought from an is but it may be practical to sync an OUGHT to an IS, i.e. it may be beneficial to all involved.

If I've understood you correctly, you're saying that morality is essentially an agreement about how to behave, based on the fact that mutual moral agreements are benefical for all parties involved. This certainly explains how morality comes to be in the first place, but it doesn't explain why we shouldn't hurt a baby, a mentally disabled person or a dog, none of whom can confer any reciprocal benefit on us. So, our morality is more complicated than that, generally.

Historically speaking babys, dogs and mentally disabled have been hurt under culturally dictated moral codes. In certain groups of world population the hurting of dogs and the mentally disabled sadly enough is still a common practice. If you wanna explain that in other groups these things are seen as morally wrong you should look at the way how stability in these societies is ensured by moral agreements.