This question comes up all the time. Let’s start with the bible and see what it says about this subject. In Genesis 4:16-17 we read:
“And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.”
So Cain knew his wife in the land of Nod and they had a son named Enoch. Notice the bible does not say that Cain found his wife in the land of Nod. The bible says that he knew her there. This means that they had sexual relations so that she would get pregnant. They were together before they arrived in the land of Nod.
So who did Adam’s sons marry?
We read in Genesis 5:4, “And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:”
So after Seth was born to Adam, Adam went on to have many more sons and daughters over a period of 800 years. The book of Genesis tells us that Adam lived a total of 930 years. He could have had a ton of children in 930 years! So the simple answer is that Adam’s sons married their sisters.
Adam’s sons married their sisters because, first of all, there was no other choice! They were the only people in the world.
Also, there were no laws against it until 2,500 years later. There was no need for the law till then. They did not need laws against this to start out with because the human race had virtually no defects. It was perfect at the beginning. But over the next 2,000 years it slowly became a problem, so God gave the law prohibiting the marrying of sisters in Leviticus chapter 18.
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