There is a natural law as well. But there was a law of God given from the very beginning in the Garden.
I understand your point and agree. They had natural law and the revelation they had been given up to that point concerning God and how to please Him. It makes me want to read Genesis with an eye to seeing what each patriarch knew about God and His ways up to Moses and the Law. It would be a fascinating read.
I do believe in natural law, but philosophically I must differ to many scholars who claim natural law as their guide. This includes Rousseau, Locke, and the Founding Fathers of America like Madison and Jefferson. The Word tells us you can derive God's existence from nature, specifically His eternality, His Divinity, and His omnipotence (Romans 1:18-23). This is enough for all people to reverence and thank such a God.
Acts 17:26-27 tells us that God wants all people to seek Him so as to find Him, since He is not far from us.
26 And he made from one man every nation of mankind to live on all the face of the earth, having determined allotted periods and the boundaries of their dwelling place, 27 that they should seek God, and perhaps feel their way toward him and find him. Yet he is actually not far from each one of us,
There are also some basic moral points you can draw from nature, such as the conclusion homosexuality is against nature (Romans 1:26-27). Another point is that nature should make us stand in awe and fear of God our Creator (Psalms 65:5-8).
I just don't see much more than these two broad points though. God and some aspects of Who He is, enough to want to seek Him, thank Him, and fear Him. And, some very basic aspects of morality, which unfortunately the modern world truly does not even understand these points anymore.
I agree though, before and probably for a time after the Flood, humanity must have had the knowledge of good and evil from their ancestors back to Adam and Eve, or from Noah a preacher of righteousness. Then at Abraham we have progressive revelation of Who God is and how to please Him, instead of only natural law and whatever knowledge of good and evil survived from the Flood.