Admittedly, the idea of natural law repulsed me before I could even give it fair thought. God-given rights? Gag me. My atheist leanings gravitated toward positivism, where humans/government create laws and grant freedom. But I was being hasty.
Natural law must not be confused with theology, and instead must be examined through a secular humanist lens. Indeed, natural law mirrors the fundamentals of basic human rights. By virtue of being humans, we are entitled to specific liberties (and I would also argue that animals and nature also have certain rights...but that is another discussion)
More importantly, the United States Constitution does not grant any rights--it merely specifies that there are certain rights which the government cannot take away. If our own government does not give us rights, it is clear then that liberty precedes government.
Natural law is also crucial because, in a purely positivist and democratic framework, the majority would lawfully be able to take away certain rights. In my book, despite mob mentality, certain rights should never be taken away. These rights can be viewed as basic human rights, or as rights derived from natural law.
Perhaps in another law, we can argue about which rights are "natural" and which are not. Tricky, as this is not written down in sea foam. But fun to think about nonetheless.