While some understand the reference to Adam in Genesis to be a general reference to mankind as a whole or the creation of more than one couple, most conservative scholars reject such a view and understand the Genesis account to refer to the creation of a literal Adam and Eve as a single couple. This is further supported by the NT. For instance Paul understood the OT to refer to a literal Adam and Eve (see Rom. 5:14; 2 Cor. 11:3; 1 Tim. 2:12-13). He clearly understood the reference to Adam and Eve to the first man and woman.
As to incest, it was not considered a sin and was not prohibited for Adam and early man. If the race was to populate and fulfill the command of Gen. 1:28, there is little doubt that Adam’s sons and daughters had to have married their own sisters and brothers if the race was to populate the earth, but due to the purity of the race as evidenced also by the long length of life, there were no adverse effects as we see happening today. Gradually, as the effects of sin took its toll on the human race, marrying one’s own sister, etc., began to create hereditary problems.
Here is Ryrie’s comment on this issue from his book Basic Theology which I would highly recommend.
Though by many inerrantists the question of where Cain got his wife would not be considered a problem at all, this question is often used by those who try to demonstrate that the Bible is unreliable in what it claims. How could it claim that Adam and Eve were the first human beings who had two sons, one of whom murdered the other, and yet who produced a large race of people? Clearly, the Bible does teach that Adam and Eve were the first created human beings. The Lord affirmed this in Matthew 19:3-9. The genealogy of Christ is traced back to Adam (Luke 3:38). Jude 14 identifies Enoch as the seventh from Adam. This could hardly mean the seventh from “mankind,” an interpretation that would be necessary if Adam were not an individual as some claim. Clearly, Cain murdered Abel and yet many people were born. Where did Cain get his wife?
We know that Adam and Eve had other sons and daughters in addition to Abel, Cain, and Seth (Gen. 5:4), and if there was only one original family, then the first marriages had to be between brothers and sisters. Such marriages in the beginning were not harmful. Incest is dangerous because inherited mutant genes that produce deformed, sickly, or moronic children are more likely to find expression in children if those genes are carried by both parents. Certainly, Adam and Eve, coming from the creative hand of God, had no such mutant genes. Therefore, marriages between brothers and sisters, or nieces and nephews in the first and second generations following Adam and Eve would not have been dangerous.
Many, many generations later, by the time of Moses, incest was then prohibited in the Mosaic laws undoubtedly for two reasons: first, such mutations that caused deformity had accumulated to the point where such unions were genetically dangerous, and second, it was forbidden because of the licentious practices of the Egyptians and Canaanites and as a general protection against such in society. It should also be noted that in addition to the Bible most other legal codes refuse to sanction marriages of close relatives.
But here is another issue to consider. If one accepts the evolutionary hypothesis as to the origin of the human race, has that really relieved the issue of incest? Not unless you also propound the idea of the evolution of many pairs of beings, pre-human or whatever, at the same time. No matter what theory of the origin of the human race one may take, are we not driven to the conclusion that in the early history of the race, there was the need for intermarriage of the children of the same pair?