The question has been raised, Where did Cain get his wife?
To answer the question, Cain married his sister. The reason that we all are taught to avoid marrying close relatives is because of the probability of passing on genetic mutations. At the time of Cain, the number of mutations being carried was much lower, so there was no reason to prohibit the close marriages. These became unlawful in the time of the Law, around 1500 B.C. When you read through Genesis, you see several marriages that are to near relatives, like cousins.
The Bible doesn’t directly say where Cain got his wife because it is fairly obvious. If Adam and Eve had other sons and daughters (which they did, see Gen. 5), these were the only other people on earth.
Bro. Doug Hibbard
Calvary Baptist Church--Loving, Sharing, and Serving