In physics, you sometimes map one frame of reference to another. So a train may be moving at a certain velocity in a certain direction. Meanwhile, let's say a plane is flying overhead in a different direction.
For the moment, let's forget the ground. It is quite possible in physics, to figure out the motion of the plane relative to the train or the train relative the plane.
The point is, to say that the meaning of events and actions is relative to a culture is not to say that meaning is random, nor is it necessarily to say that it cannot be mapped to the meanings of events and actions in other cultures. It is to say, however, that the connections must be mapped.
One person might say, "You have thrown out the window the possibility of speaking of definite rights and wrongs." No, the certainty is in the relationship between the relative frames of reference. It is a more complex fixity and requires one to realize that you are located on only one frame of reference, but there is fixity in the coordination of the frames.
But what of the ground, you ask? Is not God's frame of reference the ground, by which we might coordinate both the movement of the plane and the train?
Yes, I do consider God's point of view to be the ground. But inevitably the person who has just discovered their own frame of reference somehow thinks they can jump on God's frame of reference. But God's way is not to teach us his point of reference but to meet us in ours.
Or they might invoke the Bible as God's frame of reference. But the Bible is actually dozens of frames of reference as well--the frame of Paul, the frame of Matthew, the frame of Isaiah, etc. Mature biblical interpretation maps the frames of each to each other and also to your frame of reference today.
We cannot get off our frame of reference in this life--ever. We can only map other frames of reference to ours.