What does the Bible say about having a marital/sexual relationship with a close relative?
There are two reasons why we should not have a sexual/marital relationship with a close relative. The scientific reason is the threat of congenital defects in the offspring, a reason that is often enforced by civil law. The biblical reason has to do with morality and the overall well-being of the extended family. While civil laws usually do not address this aspect, we should always keep in view the biblical wisdom.
When God made Adam and Eve in the Garden of Eden, they were genetically perfect. They had no congenital defects, no diseases lurking in their DNA. Such issues came into the human race gradually, over the course of several thousand years. It's possible that the environmental changes after the Flood had a part in accelerating cell damage; perhaps people were exposed to more cosmic and solar radiation as the drastic decrease in ages after the Flood suggests. Adam and Eve's children married their full siblings. Two thousand years later, Abraham was Sarah's half-brother. There were many marriages between cousins in the Old Testament, including those of Isaac, Esau, and Jacob. In fact, marriage between first cousins is still allowed in some states in the U.S. And a study in the Journal of Genetic Counseling in 2002 showed that children of first cousins have only about a 2–3% higher risk of birth defects than normal—a number comparable to the defects in a child born of a 41-year-old woman as opposed to one 30 years of age.
Over the course of history, societies have progressively banned marriages and sexual relations between descendants (father/daughter; grandmother/grandson; etc.) and siblings because of genetic issues with more direct inbreeding. The bans were made easier to accommodate with the invention of the bicycle and then the car—suitors were able to go farther distances to find potential mates. These taboos tend to be enforced by law. (It could be argued that Genesis 2:24 indicates that marriage between children and parents was never allowed by God.)
Although science gives a powerful motivation to avoid marriage among close relations, the Bible's laws on incest do not address genetic anomalies or congenital defects. The laws in Leviticus 18 suggest another motivation. These are the relationships God forbade to intermarry:
Mother/stepmother and son (vs. 7–8)
Sister/half-sister/step-sister and brother (vs. 9, 11)
Grandchild and grandparent (vs. 10)
Aunt and nephew (vs. 12)
A man and his uncle's wife (v.14)
Daughter-in-law and father-in-law (vs. 15)
Sister-in-law and brother-in-law (vs. 16)
A woman and her daughter and the same man (vs. 17)
A woman and her granddaughter and the same man (vs. 17)
A woman and her sister and the same man (vs. 18)
The reasons for the restrictions are moral and relational, not health-based. Each of the above relationships was defiling (Leviticus 18:24). It was disrespectful to "uncover the nakedness" of one's parent or child, to include having sex with those relations' spouses. To have sex with a stepmother, aunt, daughter-in-law, or sibling was to upset the delicate balance of relationships in the family, dishonor the participants and their spouses, and engage in immorality. The two exceptions were if a man's wife died, he could marry her sister (vs. 18), and if a man died childless, his brother was expected to marry the widow and provide an heir. In either case, death ended the original marriage, allowing for remarriage and averting relational rivalry.
Believers in Jesus Christ are not under the Old Testament Levitical law, but the moral principles of the law are still in effect. Decisions regarding marriage to a relative should follow several factors: Legality. The Bible never prohibits marriage between first cousins, for example, but certain states and countries may have laws regarding such marriages. Health. Some states require marriages between first cousins to be sterile. Honor. Would the relationship be seen as disrespectful or dishonoring to anyone involved? Will this marriage affect the peace of the extended family? Morality. With the exception of marriage to an in-law after the death of a spouse, the relationships listed in Leviticus 18 are immoral.
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