Where did Cain's wife come from?

      Due to the fact that all people came from Adam and Eve (Genesis 2:21-23, 3:20), Cain would have had to marry one or more of his sisters (Genesis 5:4). He didn't have children through Eve because he had his own wife (Genesis 4:17). Since it is likely that Adam and Eve were genetically perfect (Genesis 1:27, 5:1) and their offspring lived nearly a thousand years in those days (Genesis 5:5, 5:8, 5:11, 5:14, 5:17, etc.), then their offspring were in no immediate danger of harmful mutations. It wasn't until after the flood and the re-population of the earth by Noah's family that incest was forbidden (Leviticus 18:6, 18:9, 18:11, 20:17, 20:19), and it was through Seth, not Cain, that the earth'c current population came about (Genesis 5:3-30, 1 Chronicles 1:1-3).

        When God created man, he made him in the likeness of God. He created them male and female and blessed them. And when they were created, he called them "man" [Hebrew adam]. When Adam had lived 130 years, he had a son in his own likeness, in his own image; and he named him Seth. After Seth was born, Adam lived 800 years and had other sons and daughters. Altogether, Adam lived 930 years, and then he died. (Genesis 5:1-5)

      Seth, too, most likely married his sisters, since he was the third child of Adam and Eve and Abel didn't live to have children. It is unlikely that he married any of Cain's daughters since Cain and his descendants were driven from Eden (Genesis 4:13-16). The Bible lacks detail, however, about Seth's wife. In those days they were also in no hurry to have children, since they were usually over one-hundred years old before beginning a family (Genesis 5:6, 5:9, etc.). It wasn't until the days of Noah that men began to increase in number (Genesis 6:1-8). Even then, men still married their sisters (Genesis 20:12--Abraham was married to Sarah, his half-sister).