What are the five basic inclinations of man according to St. Thomas Aquinas?
1. To seek good, including the highest good, which is eternal happiness with God.
2. To preserve himself in existence.
3. To preserve the species - that is to unite sexually.
4. To live in community with others.
5. To use his intellect and will -- that is, to know truth and to make his own decisions.
These inclinations are put into human nature by God to help man achieve his final end of eternal happiness from these inclinations we apply the natural law by deduction: Good should be done; this action is good; this action therefore should be done.
from 50 Questions on the Natural Law by Dr. Charles Rice, Professor of Law University of Notre Dame
That is why living a virtuous life is the means that one uses to seek the highest good. That is precisely why moral relativism is contrary to the natural law. It is the product of the evil one, who desires that we never find true happiness by deceiving ourselves into thinking it is possible without God. Precisely what Satan told Adam & Eve so many years ago. Notice that the most perfect environment for following those inclinations is the family. What moral relativism is doing is in every sense destroying family. Satan loves to have us all on the sideline taking advice from ourselves not subjecting to familial review. That way sin becomes a real gray area and morality a very subjective thing. Modern thought is anti family, anti Christian , anti Trinity, and very destructive in isolating people and their thoughts, kind of what Satan tried to do with Jesus in the desert. How did Christ answer him, when He was tempted? By using Sacred Scriptures, exactly where we should be.