You bring up some good points. Let's bake this cake now.
I don't support moral relativism because it has problems as you have said. But my position is close to that.
I support moral particularism. Which is to say that morals are not absolute. It's probably a small distinction, but I make it because I don't go as far as the moral subjectivity advocates go.
So here's the way I look at it.
Rape is wrong 99.999999999% of the time because it falls higher on the scale of moral axioms.
I think what humans do is tacitly embrace a set of de facto absolutes. This is to say that people treat morals as being absolute for practical purposes because it helps them survive better.
If I acknowledge the fact that morals are not absolute, then I also must also acknowledge that I can never justify any moral position logically.
However, I need morals to function pragmatically in reality, so I do not have this luxury.
Therefore, I am forced to treat them as absolute during the 99.999999% of time that they work and only in that moment that they fail do I apply moral particularism.
Do note that I don't take on de facto absolutes because I think they're true. I do it because it's pragmatic and I am forced to due to my inability to take in all the information universe simultaneously.
No I so believe in a type of absolute. I believe that a moral situation can be judged as 100% right or wrong assume we know enough about it. This is where I differ from the relativist. The relativist would say that it's only moral because of the axioms of that situation. I would argue that those axioms couldn't have been otherwise in that situation.
So I think the reason that rape seems to always be wrong is because it falls higher on the hierarchy of axioms. Pain/Pleasure and Life/Death seem to be the most popular and abundant axioms in society. Rape violates this and therefore it can never be morally correct unless the act of raping somehow stops you from committing an even more immoral act in the process.
Just to be clear, I don't think a real world situation could exist where rape is moral. I think it can only happen hypothetically. Not because it's impossible to have happen, but rather because our moral axioms will always steer us away from such situations. So functionally speaking. The Rape Moral will most likely be a de facto absolute that one would never have to address as a particular.
onto the evolution side.
I've heard what you said about evolution from other people. This "ingredient" makes sense if you're assuming that evolution has the goal of making us moral. Sadly, this is not the case.
You might have heard before that evolution has no goal. This is true. Evolution doesn't care if we're raping people. Sad but true. Evolution is just a broad term for gene mutations mixed with environmental factors (natural selection)
So the fact that morals are not universal is actually supported by evolution in my opinion.
For instance, if we live in a world where everybody gets their morals from the same source. Then at the very least, we should all have the same axioms across the board. Even if we interpret them differently. As you've rightly pointed out, this is not the case. Some people have axioms that seem immoral to us but moral to them. But when you consider that evolution has random mutations, this actually makes sense.
No matter how strong the moral genes that keep us alive are, there is always a chance that a person with normal moral axioms will produce a child with abnormal moral axioms. Since this always stands a chance of happening, this means that no amount of evolution can ever lead to permanently universal morality. It can only lead to a majority which is what we see in society.
In your slave analogy, I would agree that the slave is being tortured. I would also agree that it's wrong because of the particulars. Because the person is doing it for their enjoyment of torture, Because the person is a poorly treated slave who is being tortured much to their dismay, all the particulars match up and I'm not even sure there's a reason to defect from de facto absolutes from here. Owning people is generally wrong. There could hypothetically be cases where it's not wrong, although such examples likely wouldn't happen in a practical sense. But let's say that we live in a world where there is no money and food is a problem. Let's say that there is no laws in the land either to resolve civil disputes and lets say that for some strange reason, the only thing that people respect is some kind of primitive property law. So in this case, there could be times that it is moral to own a slave.
One more simple example would be living in biblical times and buying a slave to save them from slavery. Ironic, but it actually is not immoral. If I technically own them, but never abuse the power and let the "slave" live any life they choose, then I am not violating any of their human rights. Because of the law of the land, my slave would still have to live with me to keep up appearances because if I was simply to free them, then they could easily get enslaved by somebody more malevolent.
So in this case. I am making a person a slave to protect them. I am not hurting them nor am I using them for labor nor am I controlling them in any way.
It's the equivalent of marrying somebody to get them out of a dangerous country. I'm just taking advantage of the laws of the land to do the most good possible with the crappy hand that we've all been dealt.
With rape, I don't see anything like this, but I think that's because rape violates one of the higher morals which brings me to my secret next point ;)
There may be at least one moral absolute. I cannot conclude this with confidence, but theoretically, if there is even one absolute moral value, then that means there's at least one absolute axiom. There could be more, but we'd be pushing our luck to suggest it.
Pain/Pleasure or life/death seem like the best candidates for this absolute, but like I said, we have no way of knowing. It could be the case that all morals are actually absolute, but there is a methodology that allows absolute morals to be handled in a particular or relative manner. This is all hypothetical.
Now back to evolution. It actually turns out that cannibalism and rape are actually punished by evolution. Cannibalism is the easy one to debunk because it gives you a disease that kills you. This is obviously in line with evolution. Rape is more subtle, but one reason it fails in evolution is because of inbreeding. Obviously not every rape is a case of inbreeding, but more of them fit this than any human would emotionally care to admit. (We are frail beings after all)
Also, rape causes harm which at a primitive time, would have been met with tribal violence to check it. So this is how rape would have been regulated in early times. Do note that the rape gene is far less common and does not produce offspring with as much solidarity because rapist animals are much less likely to produce offspring because unwilling recipients don't have the biological triggers that help the sperm along like in consensual sex.
Lastly, it makes sense that rape is still a problem in modern society because of previous points I have made and the fact that we don't kill rapists. We just put them in jail for about 5 or 10 years which gives each rapists a handful of chances to keep spreading their genes.
In regards to what you said about morality evolving, I'd like to be clear that I don't think our biological cues are automatically virtuous simply because they promote human survival. Humans simply have a bias towards it because we like our survival. There's nothing intrinsically right or wrong about it in my opinion.
This is my grand caveat. We have to humble ourselves and realize that morality was literally made for us and acknowledge that we a nothing more than mere ants on the cosmic anthill. Our morality is a tool and we have to treat it as having imperfections like every other tool.
I await your thoughts. Do not hesitate to quote if it helps you make good points.
I can't wait to see what this cake turns out like :)