What does the Bible say about having a marital/sexual relationship with a close relative?

TL;DR:

The Bible forbids sexual or marital relationships with close relatives to protect family integrity and honor God’s design for purity. While early marriages between siblings were allowed before genetic risk, both Old and New Testaments affirm that such unions are now immoral.

what does the bible say?

We should not have a sexual/marital relationship with a close relative for two reasons: The scientific reason is the threat of congenital defects in the offspring, a reason 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.

The Bible reveals that while close-relative marriages were once necessary in humanity’s earliest generations , God later set clear boundaries to protect family relationships and preserve moral purity (Genesis 4:17; Leviticus 18:6). Leviticus 18 forbids sexual unions within the immediate family, not merely for genetic reasons but to honor God’s design (vv. 6-18).The New Testament upholds these principles by condemning incest and calling believers to holiness (1 Corinthians 5:1; 1 Thessalonians 4:3-5).

from the old testament

When God made Adam and Eve in the Garden of Eden (Genesis 1:27), 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. This is why siblings or other close relatives could intermarry with no genetic problems (Genesis 4:17).
The environmental changes after the Flood (Genesis 6-9) may have 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 (Genesis 4:1-2). About two thousand years later, Abraham married his half-sister Sarah (Genesis 20:12). The Old Testament also records many marriages between cousins, including those of Isaac (Genesis 24:15-67), Esau (Genesis 28:6-9), and Jacob (Genesis 29:10-30).
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 but that God deems such relationships unclean (Leviticus 18:24) The laws in Leviticus 18 suggest another motivation. God forbade intermarriage between mother/stepmother and son (Leviticus 18:7-8); sister/half-sister (and possibly step-sister of no biological relation brought up as one's father's daughter) and brother (Leviticus 18:9, 11); grandchild and grandparent (Leviticus 18:10); aunt and nephew (Leviticus 18:12); a man and his uncle's wife (Leviticus 18:14); daughter-in-law and father-in-law (Leviticus 18:15); sister-in-law and brother-in-law (Leviticus 18:16); a woman and her daughter and the same man (Leviticus 18:17); a woman and her granddaughter and the same man (Leviticus 18:17); a woman and her sister and the same man Leviticus 18:18).
The two exceptions to intermarriage within families were if a man's wife died, he could marry her sister (Leviticus 18:18), 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.

from the new testament

The New Testament does not give a detailed list of forbidden close-relative relationships like the Old Testament does, but it upholds sexual purity and condemns incest as sinful. Paul rebukes the church in Corinth for tolerating a man having a sexual relationship with his father’s wife and commands them to remove the offender (1 Corinthians 5:1-2).
Marriage is affirmed as honorable between a man and a woman (Hebrews 13:4), and believers are called to avoid sexual immorality of every kind (1 Thessalonians 4:3-5). Sexual relations within close family ties are outside God’s design for marriage and holiness.

implications for today

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. 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.) The bans were facilitated with the invention of the bicycle and then the car: suitors could go farther to find potential mates.

Christians 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, such as legality, health, honor, and morality. Regarding the law, the Bible never prohibits marriage between first cousins, but certain states and countries may have laws regarding such marriages. Health has also been a concern. Some states require marriages between first cousins to be sterile. Honor comes into play as well; would the relationship be seen as disrespectful or dishonoring to anyone involved? Will this marriage affect the peace of the extended family? Finally, the morality of the relationship matters.. With the exception of marriage to an in-law after the death of a spouse, the relationships listed in Leviticus 18 are immoral.

understand

Early humanity was allowed to have close-relative marriages before genetic risks developed.
Leviticus 18 forbids marital relationships with close relatives to protect family integrity and honor God’s design.
The New Testament condemns incest and calls for sexual purity.

reflect

How does God’s design for marriage challenge the way you think about family boundaries and relationships?
How can you honor God’s call to purity in your thoughts and relationships?
How do the moral principles behind God’s commands help you guard your heart and protect your family’s integrity?

engage

How can Christians explain why God shifted from allowing close-relative marriages early in history to forbidding them later?
How can the moral principles of Leviticus 18 guide our understanding of sexual purity today, and what do they teach us about God’s character and plan for humanity?
What can the church learn from Paul’s rebuke in 1 Corinthians 5 about maintaining holiness within the body of believers?