If there were only two people on Earth: Adam and Eve, then for the human race to “fill the Earth” at one point there must have been a brother and sister who would marry. This is not a perverse establishment by God since in that time there would have been no laws and problems with brother and sister marrying. This eventually changed certainly, but in the beginning there was nothing morally incorrect with that just like in the time of Abraham there was nothing wrong with his marrying his half-sister (Genesis 20:12), which marriage God blessed (Genesis 17:16), but later those kinds of marriages were restricted in the Law of Moses (Lev. 20:19).
Genesis 4:17 – Where did Cain get his wife?
