We read in Genesis Chapter 4 about Cain murdering his brother Abel, and then Cain left the area with his wife and went to the land of Nod. Where did Cain get his wife? The answer is simple: he, and all of his siblings, married their female relatives. Now, many people may not like this idea, and even RTG could not fathom marrying any close relative, but the fact of the matter is that there were no other people on the Earth at this time in history except for those directly descended from Adam and Eve. At this time in history, everyone was under the curse, but the harmful effects on the DNA had not spread to a dangerous level, so marrying within the family would not have produced genetic disorders and other problems like it would today. When the population on Earth had grown to a large number, God put a stop to inter-marrying (Leviticus 18:6), although technically we still marry our relatives.