This is one of the most common questions we receive. If Adam was the first man, directly created by God, and Eve the first woman, also a direct creation, and only three sons are mentioned — Cain, Abel, and Seth, it is but natural that many ask: "Where then, did Cain get his wife?"
Did Cain have a wife? Yes, it is written in Genesis 4:17: "And Cain knew his wife; and she conceived, and bare Enoch..."
Cain had a wife, and a son.
For that matter, where did Seth get a wife? Seth is a direct ancestor of Noah, of Abraham, of David, and of Christ. So Seth, as well as Cain, must have had a wife.
The answer is found in the genealogy of Adam, in Genesis 5. It is written in verses 4-5: "And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters: And all the days that Adam lived were nine hundred and thirty years: and he died."
There is the answer. Adam begat sons AND DAUGHTERS. God had blessed Adam and Eve, and said unto them: "Be fruitful, and multiply, and replenish the earth..." (Gen. 1:28). All human life started from them. How could they be fruitful and multiply, unless they had BOTH sons and daughters, and their sons married the daughters?
The answer is simple. Both Cain and Seth married their sisters. There undoubtedly were many of them even before the birth of Seth.
Adam and Eve, as God had proposed, were fruitful! After the birth of Seth, Adam lived EIGHT HUNDRED YEARS, and "he begat sons and daughters." The sense of the passage is that he continued begetting sons and daughters. That is a long time — as long as though some man and his wife living back in the middle ages — way back in the 1100's had continued to live and continually bear children ever since, clear up to now — the 1900's! Adam was 130 years of age when Seth was born (Gen. 5:3). After that — after he was 130 years of age — Adam continued begetting sons and daughters eight hundred years!
It's almost impossible for our minds to grasp, in this fast-moving, short-lived world of one or two children to a family, how many children Adam and Eve probably had during their great span of life, nearly a thousand years. It's hard for us to realize that Adam lived almost ONE-SIXTH of all the time from his creation until now. Yes, there must have been plenty of young women for wives for Cain, and Seth, and all their many other brothers, as the hundreds of years dragged along!
Let us also remember that it was not wrong to marry a sister or brother in the beginning. No physical harm would result. Over two thousand years later — in the days of Abraham — a man could still marry a half sister. It was not until the days of Moses that God forbade brothers to marry their half sisters (Lev. 18:6, 11).
Back in those antediluvian days, when people lived seven, eight, and nine hundred years, they did not age as we do today. They were able to continue bearing children, undoubtedly, for hundreds of years. After the flood, due to human dissipation and living contrary to God's revealed right laws of living, the life span of man became shortened.
By the time of Abraham, Sarah was considered too old to bear children at age 90, and Abraham thought it impossible for him to become a father again at the age of 100 (see Gen. 17:17), Isaac was born as a direct miracle, But back before the flood, and in the days of Adam, it was different — and would be today, had not men departed from the laws of living revealed by God. May God help us to STUDY to learn those laws and start living by them today.