Prohibited Sexual Relationships
The Bible--concerned with sanctity in all aspects of life--prohibits incest, adultery, and a number of other sexual relationships.
The above biblical passage opens and closes with admonitions against transgressing God's commandments in the area of forbidden sexual activity. The main section of the chapter can be divided into three classes of forbidden sexual activity: incest (verses 6‑16); unions with women who are closely related to each other (verses 17‑18); and other forbidden sexual activity, including adultery (verses 19‑20, 22‑23). The underlying concern of the prohibitions enumerated here is the continuity of the Israelite family over successive generations.
Biblical law forbids the following sexual relationships:
1. Between mother and son, even if the son was not her legitimate child.
2. Between stepmother and stepson, even if the stepmother is no longer married to her stepson's father.
3. Between brother and sister, or half-brother and half-sister. However a man is permitted to marry his stepsister (i.e., the daughter of either stepparent by a previous marriage).
4. Between a father and daughter, even if she is his illegitimate child, or grandfather (paternal or maternal) and granddaughter.
5. With the sister of either parent, even if that sister is not a legitimate child of his grandparents.
6. With the wife of one's father's brother (i.e., of his father's brother from the same grandfather).
7. With one's own daughter‑in‑law, even if she is no longer married to his son.
8. With the wife of one's brother or half-brother.
9. With a woman and her daughter.
10. With a woman and her son's daughter.
11. With a woman and her daughter's daughter.
12. With one's wife's sister while the wife is still living.
There is a substantial amount of commentary regarding the prohibitions enumerated above. Maimonides, always having argued that sexual relations ought to be kept to a minimum, states this as the reason why men were forbidden to have intercourse with close blood relations who live in the immediate environment and for whom they naturally care. Another reason, he posits, for the prohibition of sex relations or marriage with blood relatives is the simple motive of shame. He states that it is indeed shameful that the trunk (i.e., the father) should copulate with the branch (i.e., his daughter).
Nachmanides does not agree with Maimonides' idea that sexual relations with blood relatives were forbidden in order to restrict men in their sexual activity. If sex is repulsive and therefore should be minimized, why, he asks, did the Torah permit a man to marry even a thousand women as long as they were not his close relatives?
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