Let me clarify even further: I agree that there is a moral law, or at least the first principles of moral law which are accessible to every person who uses reason rightly. I do not think that this means every person will reason rightly.
I do not take the fallen nature of mankind to be such that it precludes individual innate knowledge of some first moral principles. In my understanding, The Fall precludes mankind's access to salvific grace in such a way that only God can restore that access.
I don't see how the Fall precludes man's access to all moral knowledge--though I will grant that the Fall significantly clouds his reasoning. Because of his clouded understanding, each man needs to check his reasoning against the reasoning of those wiser than himself and against the wisdom of the church.
Man using his God-given intellect can grasp the first principles of logic and of math. Why then not the first principles of morals?
I believe this is exactly what those opposed to natural law would argue with you on. How does a man "checking" his reasoning determine who is "wiser than himself"? If man is corrupted how do you determine who has greater understanding? What do you base your evaluation on in a corrupted state?
In your example of logic and math, these are not something that comes natural to all people. Why should we think the first principles of morals? While math is more rigid (although some people cannot grasp math), logical thought could rightly go down different paths. The conclusion Socrates came to might not be the same conclusion Plato came to. Whose right? Those who disagree with natural law would say that your argument proves that natural law is nothing more than ethics taught and developed in a child, just like logic and math.
I shouldn't be so hasty to just completely dismiss natural law. However, this to me, logically speaking, is an unprovable hypothesis. ;O)