Genesis 4 I believe is where the division begins because people just can’t believe any implication that siblings married. But it’s not implied… it is spoken of in later chapters of relatives marrying… Abraham marries his sister from another mother. Isaac marries his paternal cousin. Jacob also marries his cousins on his mother’s side – sisters, Leah and Rachel. So with that clarity, is it that hard to grasp that the children of Adam and Eve married each other?
Of course, nowadays there are implications to marrying family and the Bible does later on speak against this concept, so do not mistake my words for saying that this is right in this day and age.
Anyways, Genesis 4 is the chapter that introduces us to Cain and Abel, Adam and Eve’s first children and the first murder.
Cain is jealous of his brother because Abel’s offerings are favorable to God, (and you can read as to why that is here) and as a result, Cain murders Abel, opening a door to a curse.
Now you are under a curse and driven from the ground, which opened its mouth to receive your brother’s blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth.Genesis 4:11-12 NIV
Cain then proceeds to ‘leave the Lord’s presence’ and marries (4:17), he then has a son name Enoch, whom he names a city after.
My study Bible points out this: The fact that Cain names a city after his son indicates the rapid and dramatic increase in population.
In other words, though we are only told about Cain and Abel, we can assume that Adam and Eve had other children, who had other children and through whom the world populated. This can even be seen in the fact that Adam and Eve had another child, Seth, who is specifically mentioned for one reason: it will be through his descendants that the Messiah would come. His name even means ‘to place’ or ‘to set’ in Hebrew, meaning that “he was appointed to take this special place in the plan of God.” (NIV Starting Place Study Bible, Zondervan 2019).