I've tried looking at the mathematics of Genesis but I've decided that it just doesn't make any sense. Anyway, however you look at it, starting with just two people means that all of the early 'begatting' must have involved a great deal of sexual congress between close family members.
So I can't really answer your question but I can, at least, help you quickly find the relevant sections for any future questions you might want to post: If you click on the word 'Topics' (at the top left of this page) you'll find that the list on the left is expanded to show all of AB's sub-topics (such as 'Religion & Spirituality').
Chris