I suggest the book 50 Questions on The Natural Law by Charles E Rice
An article you can read:
newadvent.org/cathen/09076a.htm
Ok, I did. Here is the start:
…the natural law is the rule of conduct which is prescribed to us by the Creator in the constitution of the nature with which He has endowed us.
This is a purely religious concept. How can something be called “natural”, when it is prescribed by the supernatural?
Later it says:
Theoretically speaking, man is capable of acquiring a full kowledge of the moral law, which is, as we have seen, nothing but the dictates of reason properly exercised.
Now this would be a good starting definition (but not sufficient without further clarification), but it is contradictory to the other definition. Sure, we all can arrive (using pure reason) at the concepts of “golden rules”. These two rules are sufficient for humans to live together in peace, respecting each other and helping each other when the need arises. That is all what is needed.
But the Christian assumption does not stop here. Christians wish to extend this “natural law” to incorporate all sorts of normal, natural and healthy human behaviors by declaring them contradictory to some “commandments”. Mind you, we have a few threads in progress, in which the believers are asked to provide fully rational, secular arguments against some of these, and they fail miserably. No one is able to present a fully rational, secular argument against those sexual behaviors, which are forbidden by the Catholic Church. (These include: sex outside the marriage, homosexual love, masturbation, artificial birth control, sex without being “open” to procreation, etc… etc…) And yet, the Church maintains that these behaviors are contrary to “natural law”.
Sorry, bro, it simply will not wash. If you cannot substantiate that these behaviors are “irrational”, you have only one fall-back position, that God prohibited them. But in that case (even if you are correct, which is questionable) it becomes a religious argument, and not a rational, secular one.