Problem: If Cain married his sister, this is incest, which the Bible condemns (Lev. 18:6). Furthermore, incestuous marriages often produce genetically defective children.
Solution: First, there were no genetic imperfections at the beginning of the human race. God created a genetically perfect Adam (Gen. 1:27). Genetic defects resulted from the Fall and only occurred gradually over long periods of time. Further, there was no command in Cain’s day not to marry a close relative. This command (Lev. 18) came thousands of years later in Moses’ day (c. 1500 b.c.). Finally, since the human race began with a single pair (Adam and Eve), Cain had no one else to marry except a close female relative (sister or niece).
