Just recently I have heard a question that has been circulated among many different mediums. Just the other night this same question came up in my history class and I would like to know how to answer it. Forgive me if I have somehow missed the logic in this question, but I am writing to you for assistance. The question has to do with the fact that Adam and Eve's boys had children. The question being how did they do this if they were the first family on the planet.
"And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth. After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters" (Genesis 5:3-4).
Adam and Eve did not just have three sons: Cain, Abel, and Seth. Genesis 5:4 tells us there were other sons and daughters as well. Thus, the simple answer is that the siblings married each other.
Generally, this is met with revulsion because it has been strongly ingrained in us that close relatives should not marry each other. But the reason for this requirement is that birth defects are very high when near relations marry. Yet, this did not exist in the beginning. The creation was very good when it was completed (Genesis 1:31). After the fall of man, it began it slow decay and with the decay came genetic defects.
Recall also that Eve was made from Adam's rib (Genesis 2:21-22). In a sense, you could call this the first cloning, and yet Adam married Eve and we all descend from them.
As we read through the history recorded in the Bible we see the nearest of relatives spreading. Abraham married his half-sister (Genesis 20:12). Abraham told both Pharaoh and Abimelech that Sarah was his sister (a half-truth) and since they never stop to wonder if Sarah and Abraham were married, we can assume that by this day marriage between full brother and sister did not happen. By the time we reach the days of Moses, marriage between near relations was forbidden (Leviticus 18:6-18). However, marriage between cousins was allowed. Today, even this close of a relationship is avoided because of the odds of birth defects.