If Adam and Eve were created first, who did their children marry?

If one is to take the account in Genesis chapter one and two at face
value, then one must assume that Adam and Eve’s children did indeed marry
one another. Some have speculated that at this point in the history of
humanity, there were few if any inheritable genetic diseases, making the
marriage of siblings a relatively safe thing. Later on, for example when
Moses was given the Law on Mt. Sinai, God specifically proscribed marriage
of siblings, but this was many many generations later. By this time, due
to mutations, there were sufficient genetic diseases available in the gene
pool to make intermarriage with siblings an unsafe practice. I will admit
that I am interjecting some speculation here. We do not know for sure what
God’s reason for outlawing incest was, but we do know that one possible
reason for God outlawing incest is that it produces a much higher
incidence of genetic disease. In Exodus, God specifically declared to the
Jews that if they would follow his decrees they would not experience many
of the diseases of their contemporaries. The law against incest found in
Leviticus protected them from many such diseases. However, it may well be
that this reason for outlawing incest may not have been relevant in the
time of Adam, Eve and their offspring, which would explain why this
presented no moral issue for the children of Adam and Eve.

John Oakes, PhD