Just because two cultures disagree on a topic does not refute the thought that there's an objectively correct moral stance to take. Consider the nature of the Earth. If one person (or for the sake of this situation, one culture) subscribes to a flat-Earth model, while another to a non-flat-Earth model, that does not imply that "the true nature of the Earth must not have any objective state." To us this is obvious, but to those two folks (or cultures), the nature of the planet might remain up for debate! OP argues there's a mysterious set of objective rules out there somewhere. The argument that societies disagree on what's right or wrong, therefore there's no objective right, can't really be applied here.

To us, the flatness of the Earth can be disproven, so there is a "correct" culture. However, before this proof surfaced, that didn't mean that the flatness of the Earth was a subjective description—up to the eye of the beholder. I think the same argument can potentially be applied to cultural values. One culture might be "right!"

Check out James Rachels's Cultural Relativism if you wanna dig deeper.