Laws Concerning Sexual Immorality (Leviticus 18)
Chapter 18 continues the theme of holiness and separation. The instructions here are directed to all Israel, and no mention is made of the priesthood. Hence, the instructions are not for ritual sanctification, per se. Since the instructions regard prohibited sexual relationships, they appear to be for social holiness, that is, for producing right relationships between the basic units of societymen and women.
Sexual immorality has been a persistent problem in all human cultures. The societies of Israel's time committed a variety of sexual perversions, as do ours today. To create a holy nation, a nation whose individual and societal conduct was pure and stable, God had to make clear which sexual relationships were forbidden.
The list of prohibited relationships includes marriage between (1) parent and child, (2) stepparent and stepchild, (3) full siblings, (4) half-siblings, (5) grandparent and grandchild, (6) uncle and niece, or aunt and nephew, (7) father-in-law and daughter-in-law, and (8) brother-in-law and sister-in-law. Also, a man was forbidden to marry a woman as well as her daughter or granddaughter. God also now prohibited a man from taking his wife's sister as a second wife while his wife was alive; such a marriage would likely ruin the relationship between the sisters and produce endless rivalry and strife within the family.
These prohibitions, which are still in force, prevent destructive sexual relationships with the near of kin, prohibiting sexual relations with persons within two generations of an individual. As can be seen, these prohibitions, had they been enacted earlier, would have prohibited the marriages of any of Adam and Eve's children (per prohibition 3 above), Abraham and Sarah (per prohibition 4 above), as well as Jacob and Leah and Rachel (per the prohibition against marrying a wife's sister). No particular reason is given for the prohibitions, but medical science has demonstrated that the children of unions between near of kin, as defined by God, have a greater risk of genetic abnormalityand it is possible that this was a factor in the enactment of these prohibitions.
God also prohibits sexual relations with a woman during menstruation. While no reason for the prohibition is given, it is possible that a sensitive God gave it to provide a measure of protection for women during this often-uncomfortable period. Menstruation frequently produces mild or even severe physical discomfort, and a woman's emotional condition at this time can be fragile. Moreover, medical science has shown that sex during menstruation poses a greater risk of tissue injury or infection to the woman, as well as of transmitting blood-borne disease from one partner to another. God's giving of this law may also be tied to the special role of blood for the atoning of sin, as blood seems to be the major concern in Leviticus 20:18. Whatever the reason, God takes this matter very seriouslyin the verse just cited, where God imposed a severe penalty for violation, as well as in Ezekiel 18, where it is declared a matter of righteousness (verses 5-9).
God concludes his instruction regarding illicit sexual relations and practices by pointedly reminding the Israelites that such conduct defiles not only them but also the land. It is easy to assume that bad conduct only affects the perpetrator and those immediately around him. Not so. The moral quality of a people extends far beyond them to the very land upon which they dwell. God reminds Israel that because such abominable acts were committed by the people of Canaan, that land was going to "vomit them out." Far from being a figure of speech or a poetic device, God's warning reveals a very real moral law of the universe. Sin has a material impact on the natural world. Lucifer sinned and Scripture seems to indicate that the creation was devastated as a result. Adam sinned and the plant and animal natures were corrupted. Just so, when a nation becomes sinful, even its land is defiled. Sin affects everythingman, beast, vegetation and land.