Most people don't understand natural law.
They think it has something to do with Mother Nature.
Natural law is based on the idea that everything has its own nature.
And when I use the word "nature", I'm referring to Aristotle's definition of "nature" - the range of actions available to a creature.
The Divine Nature is Love. God is Love, precisely because each Person of the Trinity pours Himself out into the other Two Persons of the Trinity. God's personhood is totally defined by the relationships between Father, Son and Spirit. The Divine Nature is about making a total gift of self.
We are made in the image and likeness of God.
Thus, human nature is about the total gift of self as well: gift of self to God, and gift of self to the other images of God around us, other human beings.
We are said to abide by natural law only insofar as we image the nature of God, the life of God, in our own nature, in our own lives. That is abiding by natural law - living out the life of God in my own life.
It has nothing to do with Mother Nature.
Consider, for example, the infant in the neonatal intensive care unit. Nothing in that room is natural except for the baby itself. The tubes, the monitors, the saline drips, the needles, the oxygen tank, the temperature regulators - all of it is artificial, made by the artifice of man. Yet the parents who put the child into that neonatal intensive care unit are acting according to the principles of natural law. God is the Life-Giver, the one who preserves and protects the weak. Whatever we do that imitates God's life-giving preservation and protection is in accord with our nature because it mirrors the divine nature.
Now, what if the parents were to take that same child and place it on the forest floor and walk away? There is nothing more natural than the forest, with the bees, and ants, the birds sweetly warbling in the trees, the sun streaming down through the leafy branches as they are gently stirred by the wind! Yet abandoning that child into that purely bucolic setting is an unnatural act, it is a violation of the principles of natural law, because it is not in accord with the divine nature. It doesn't mirror God's love, care, and concern for human life.
When someone commits a sin, they become more "natural" in the sense that they become more like an animal. Do we not say of a convicted serial rapist/ax murderer, "He's an animal! Lock him up and throw away the key!"
There are many ways to define sin. One definition is very appropriate to this discussion. Sin is following our natural impulses, instead of our supernatural impulses.
Natural law concerns itself with how we as human beings live out the life of Christ, of God, in our own bodies. The natural law cannot be followed without supernatural grace because we are, ultimately, called to be not just natural creatures, but supernatural creatures, sharers in the divine nature.
If that point is not understood, if the reason for natural law is not grasped, then whatever is written about natural law is utter nonsense.