What do you mean by "natural law"? Laws of physics I understand. Natural law against incest leaves me puzzled. What was the basis for / origin of, this “natural law”. Also, are you implying that marriage was around at the time of Adam? If so, could you give me a reference?
Read some Aquinas. Natural law is the moral order proper to human nature; the rights and wrongs that human beings, at least unfallen ones, would know instinctively.
Also, are you implying that marriage was around at the time of Adam? If so, could you give me a reference?
The book of Genesis refers directly to Eve as Adam's "wife". There's also a Hebrew apocryphal book (Book of Jubilees, I think?) which names all of Adam & Eve's sons and their sisters whom they took as wives.