Q: #103. Why did God allow incest in the Old Testament?
By: Steve Shirley
A: Adam and Eve were the first people on this planet. We are ALL descended from them. When God created Adam and Eve, He said, "Be fruitful and multiply, and replenish (fill) the earth" (Gen 1:28). They had many sons and daughters. Obviously, there was no way for the world to be populated unless these brothers and sisters married each other and had children.
God allowed this to occur for about 1500 years, and then He made a law prohibiting it (Lev 18:6-18), and made a punishment for those who continued it (Lev 20:11-21).
After 1500 years, there were plenty of people on the Earth, and this practice did not need to continue. However, it is also quite likely that the main reason that God ordered it stopped was because of the dangers that could occur to the offspring.
When Adam and Eve were created, they were genetically perfect (very good: Gen 1:31). Once they brought sin into the world, imperfection and genetic defects began to occur as the result. With each generation, these genetic defects continued to increase until a point came when it was a danger for brothers and sisters to have children. This is why we are not allowed today to marry a brother, sister, or close cousin today, because of the greatly increased danger to offspring.
I can't really explain the whole science behind this very well, but in short, we all have genes that we inherit from our parents, and these genes have mistakes in them. When identical bad genes are combined from two different people in a child, it causes the child to have a deformity. Since a brother and sister have the same parents, they would also be likely to share the same bad genes. Therefore, if they had a child together, there is a greatly increased likelihood that their child would be deformed. The more closely related, the more likely the problem.
Today, the idea of brothers and sisters marrying seems disgusting, but there is nothing inherently wrong with it. In the beginning of the world, God didn't have a problem with it, nor did the people. It should be noted, however, that God did not EVER condone parents and children mating. In fact, when this did occur in the Old Testament, in (Gen 19:30-38) with Lot and his daughters, the result was sons from each daughter who were VERY wicked people (the Moabites and the Ammonites).