Is marrying close relatives forbidden?

Some verses state that people married their close relatives: Genesis 20:12 (see also Genesis 17:16), and Exodus 6:20.  Furthermore, Cain must have married a close relative—probably his sister (being the son of Adam and Eve).  However, later, God said marrying close relative was wrong (Deuteronomy 27:22; see also, Leviticus 18:9, 12; Leviticus 20:17, 19).

Response

Inter-marrying close relatives was wrong after God said it was, but before that time, it was acceptable (and even necessary).

For instance, Cain must have married a close relative (probably a sister).  But today, such marriage would likely result in deformities.  This is because early on, the human gene pool had a lot of diversity in it (and was near-perfect just after the curse).  Today, because of the curse of sin and the thousands of years of building mutations in the human genome, and due to the loss of some variety within individual lineages, we need to marry a non-close relative so our genetic errors “cancel out” each other.  Close relative are likely to have similar genetic mutations which will express themselves in the offspring, but this was not the case early in creation.

In fact, the timing of the ban on marriage of close kin supports the accuracy of the Bible from a genetic standpoint.  The Bible is accurate in every way, including scientifically, and more specifically, genetically.