I've been waiting, and finally somebody actually got around to asking the question: "Where did Cain get his wife?" (This one comes from a reader in Virginia.)
According to the story in Genesis 4, Cain kills his brother Abel in a fit of jealousy. God confronts Cain and he responds with the famous question, "Am I my brother's keeper?" God condemns Cain to a nomadic life and Cain cries, "Today you have driven me away . . . I shall be a fugitive and a wanderer on the earth, and anyone who meets me may kill me. . . . Then Cain went . . . and settled in the land of Nod, east of Eden. Cain knew his wife, and she conceived and bore Enoch; and he built a city and named it Enoch" (Genesis 4.14, 16-17).
Here then is our concern: according to the genealogy of humanity, the only existing humans on the earth at this time were Adam, Eve, and Cain. So, where did the people in the land of Nod come from?
There are at least three possible answers to the dilemma. The first answer is probably the most common "pat" answer I've heard. There is no time period enumerated between Cain's exile to Nod and his marriage and, since Adam purportedly lived some 930 years and fathered "other sons and daughters" (Genesis 5.4), it is possible Cain married one of his sisters at a later date. Of course, this conclusion offers no suggestions of whom Cain feared in Nod at the time of his curse, nor does it take into account the law (are God's laws timeless?) that a brother cannot take a sister as a mate (Leviticus 18.9)--not that Cain was particularly law abiding.
A second answer is suggested by the double creation theory. In this thought, Genesis 1 accounts for a general creation of the earth, and chapter 2, a second, more specific creation wherein Adam and Eve were created and placed in a garden paradise. The first creation yet existed outside the Adamic paradise. In this notion other humans existed upon the earth beyond of the geography of Adam and his family, thus Cain could have been rightfully frightened to leave the security of his home.
The third answer takes into account the biblical story as truth beyond history. However, if the story of Cain and his plight isn't a historical event, then we must account for it through its intended meaning.
Since most of the early biblical stories are generally etiologies, that is, stories of origins, it is likely this account is an etiology as well. In the story of Cain and Abel it is possible Abel represents the shepherding clans, whereas Cain represents the agriculturists. It has been suggested the story is a metaphor for the fleeting history of the nomadic lifestyle of the wandering shepherd tribes and the comparative stability of the agriculturist. However, this possibility seems contrived and convoluted.
More likely is the notion that this story speaks of the comparative "evils" of city life, since Cain is forced to give up his agricultural roots and establishes the first city (for more information regarding the Israelite condemnation of ancient cities, cf. Genesis 13.12; 19.1-30). Further, Cain's son Lamech, who resides in the city, follows in his father's footsteps as a ruthless murderer (Genesis 4.23-24).
In any event, the biblical writer doesn't seem concerned with the details of the story, such as where Cain got his wife or who the enemies of Cain might be. The purpose of the story wasn't to set forth an accurate historical account, but to teach the generations that we are our brother's and sister's keepers, that potential threats to societal life must be separated from society (cf. Numbers 35.11-12), and that the ideal of life is to live at harmony with the land and nature, not to be confined within the boundaries of a city.
