But natural law is not manmade; it simply arises from nature.
Consider a triangle. A triangle is any closed-plane shape consisting of three straight lines. Anything that meets this definition will exhibit certain properties, such as having angles that sum to 180 degrees.
Now, obviously, not every triangle I draw will live up to this standard (which we can call "triangularity"). If I draw it hastily, it may not be perfectly closed, or its lines may not be perfectly straight, and therefore its angles will sum to something other than 180 degrees. We could say that an exceptionally badly-drawn triangle is a "bad triangle," because it fails to instantiate the essence of triangularity. By contrast, a triangle drawn with a straight edge and a rapidograph will come much closer to instantiating the essence of triangularity and so will be, by comparison, a "good" triangle.
It would be foolish to say that this springs from an irrational value preference for triangles with 180 degrees. Triangles simply have 180 degrees -- it is a fact arising from nature. (We can, of course, quantify it differently, say by changing how much a degree counts for; but all the relationships would be preserved as a result of this change). These are not values we make up, they are facts we discover.
That is the key finding that comes out of classical essentialism, the foundation on which the Church is built: that everything which exists can be said to have a more perfect form or essence which it resembles but imperfectly; in short, that there are norms which arise not merely from convention but from nature itself. This is no less true of material things, rather than abstractions like shapes. If a pen writes very well, I might say "this is a good pen." If my dog is very loyal and friendly (that being the purpose for which that particular breed of dog has been selected), I might say "this is a good dog." If a dramatic movie moves me to tears, or a comedy moves me to laughter, I might say "this is a good movie."
It applies to people as well: people have an essence, which can be discerned by examining the features that compose them. They have hearts that pump blood and lungs that process air; eyes to see and ears to hear. But their most distinctive feature is that they have reason to discern the truth, and will to act in accordance with it.
It is unsurprising that this is one of the first truths that civilized man discerned.