Why is rape wrong though? "hurr durr rape bad" is just repeating whatever you've been indoctrinated to believe. There's a more fundamental reason rape is bad (and this same fundamental reason underpins the reason we find a lot of other things bad).

It does fall back to "core values" though - kinda' like with math & axioms. The "why" chain of questions will inevitably lead to something like "because there's inherent value in human life", and this is the point where it breaks down because there's no logical reason to say that. You can probably postulate the contrary and end up with a completely different set of morals that may still be internally coherent but would be very alien to you. Just how you can say "in a plane, through a given point not on a given line, there is no line parallel to the given line" and end up with a weird, non-Euclidean but coherent geometry.