Abraham and some other great men in the Bible married their relatives, right?

Leviticus 18:1-19 (NIV):

“The LORD said to Moses, ‘Speak to the Israelites and say to them…You must not do as they do in Egypt…You must obey my laws and be careful to follow my decrees. I am the LORD your God…’

‘No one is to approach any close relative to have sexual relations. I am the LORD.

‘Do not dishonor your father by having sexual relations with your mother. She is your mother; do not have relations with her.

‘Do not have sexual relations with your father’s wife; that would dishonor your father.

‘Do not have sexual relations with your sister, either your father’s daughter or your mother’s daughter, whether she was born in the same home or elsewhere.

‘Do not have sexual relations with your son’s daughter or your daughter’s daughter; that would dishonor you.

‘Do not have sexual relations with the daughter of your father’s wife, born to your father; she is your sister.

‘Do not have sexual relations with your father’s sister; she is your father’s close relative.

‘Do not have sexual relations with your mother’s sister, because she is your mother’s close relative.

‘Do not dishonor your father’s brother by approaching his wife to have sexual relations; she is your aunt.

‘Do not have sexual relations with your daughter-in-law. She is your son’s wife; do not have relations with her.

‘Do not have sexual relations with your brother’s wife; that would dishonor your brother.

‘Do not have sexual relations with both a woman and her daughter. Do not have sexual relations with either her son’s daughter or her daughter’s daughter; they are her close relatives. That is wickedness.

‘Do not take your wife’s sister as a rival wife and have sexual relations with her while your wife is living…’”

Adam and Eve were created perfect. After they sinned, they lost perfection but their genes were still nearly perfect. It was necessary for their children to intermarry in order to populate the earth. They and their offspring were very healthy as is evidenced by the typical life spans of 800 to 900 years. (See Genesis 5.) Whereas today, intermarriage produces children with serious genetic defects resulting in poor health, sickness, and death.

As the population of the earth grew, it became unnecessary to intermarry. By the time the Law was given, God strictly commanded against intermarriage as stated in Leviticus 18. Any close relatives not specifically identified in those verses are still off-limits. Verse six reads, “No one is to approach any close relative to have sexual relations.”

Did some of the patriarchs marry relatives? Yes. But if you read the stories of their lives, intermarriage usually resulted in serious problems. Problems with rivalry, infertility, jealousy, grief, etc. were directly caused by marrying relatives.  Today, intermarriage is still a sin from God’s perspective, but it is also against civil law in most countries.