The law before the Law
There were some animals approved for eating and sacrifice before the flood (Gen 7:2). Apparently God gave some religious and dietary instructions to man before the Law of Moses. Prohibitions against murder and eating raw meat were reinforced after the flood (Gen 9:4-5). There must have been some instruction on sexual morality as well, because Ham was held accountable for his perverse reaction to discovering Noah naked (Gen 9:22). After the fall, God did not leave us to our tainted consciences. He gave man some specific rules about right and wrong. Conscience can be a wonderful guide (because God gave it to us), but it has never been the sole standard of good and evil. Some things are wrong because God says they are wrong. It doesn’t matter what we think. We’re not smarter than God. We can’t decide that something is right and declare it so in contradiction to what God has already declared.