There's no claim that they did. The claim is that their children married each other; brother-sister incest, not father-daughter incest.
The question then becomes whether brother-sister incest (within the context of marriage, obviously) is prohibited by natural law. (Since no law against it had been revealed at the time, only natural law could have applied.) It's not at all clear that it is prohibited.
If you want this in simpler terms, there are some thing that are wrong because they're just wrong. There are other things that we know to be wrong because God has said that they're wrong. It's not obvious that brother-sister incest isn't in the second category.
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?