Arayos: Why prohibit marriage of relatives?

Ramban (Vayikra 18:6): Concerning all near of kin to him, he should not approach them to uncover their nakedness The reason for prohibition of sexual relations with certain relatives is not stated explicitly. The Rambam (Moreh Nevuchim 3:49) says it is to reduce sexual activity and to make it less attractive and that he should be satisfied with little. These types of women which are prohibited by the Torah are those that are more likely to be present and available to him. And this reason the Rambam applies to all of them. The Ibn Ezra writes something similar that since the lust of a man is like that of an animal therefore it is not reasonable to prohibit all women. What is prohibited were those women who are present all the time. However this is a very weak explanation to say that the Torah makes a person liable for kares simply because they are with him on occasion and at the same time permit a man to marry hundreds and thousands of women. Why should it be harmful if he just marries his daughter as is permitted to non-Jews (Sanhedrin 58 :) or two sisters like Yaakov did? In fact there is no more appropriate person to marry than his daughter to an older son and then they will keep the inheritance in the family and will have children in his house in order that the land not be waste and the lust will quieted. The fact is we don’t have a traditional explanation concerning this. However according to logic there must be a deep secret of creation regarding his soul and this is included in the secret of incarnation which has already been alluded to. You should know that sexual intercourse is something which is disgusting and despised in the Torah – except for the purpose of propagating the species. If the relation doesn’t lead to children it is prohibited. Similarly that which doesn’t preserve the species and isn’t successful is also prohibited by the Torah. This is the reason for the Torah views on intercourse with relatives... Thus later on in verse 17 it says that intercourse with relatives is wickedness. In other words it is not properly marriage because it won’t be successful but represents simply evil lustful thoughts. In fact sexual prohibitions are included as statutes which are things which the king decree. And the decrees are things which occur to the king who wise in the conduct of his kingdom and he knows what is needed. And that which is beneficial he commands and he doesn’t reveal to his people except for his wise men and advisors.

Rav S. R. Hirsch (Bereishis 2:24): This verse – according to our Sages – alludes to the halachos of arayos for Bnei Noach (Sanhedrin 57b). The verse says, “Therefore a man should leave his father and mother and cleave to his wife...” This can be understood to mean that a man should distance himself from his father and mother – in other words when chosing his wife he should not seek a wife amongst his closest blood relatives. Perhaps if we examine this verse in the context of what preceded it – as suggested by its opening with “and therefore” - we can have at least a partial clarification of the reasons for the apparently inexplicable laws of arayos. The role of the wife is to be the helper (ezer) to the husband and as a result she must be compatible with him (kenegdo) and that means that she needs to complement him and therefore she must have different characteristics than her husband. In fact if they are too similar that would mean they would not only have the same virtues and but also the same faults. Thus if they got married it would only serve to reinforce the characteristics of both – whether for the good or bad. But marriage would not lead to perfection since they do not complement each other. Thus the possiblity of perfection through being complementary is only if they are not too similar and in fact have distinctly different characteristics. It is only due to having different characteristics that their joining together has the potential to produce perfection. Thus not marrying blood relatives increases the likelihood for finding a complementary mate. However this explanation of arayos only is relevant for the arayos of Bnei Noach which are based entirely on blood relatives. In contrast the arayos for a Jew are also based on kinship through marriage. Such arayos requires a higher level of explanation.

Kli Yakar (Vayikra 18:6): The reason for the prohibition of arayos according to the Rambam is in order to reduce the amount of sexual intercourse. Since these prohibited women are relatives of his and thus they are always with him. The Ramban refutes this view by noting that the Torah in fact allows a person to have thousands of wives. Therefore the Ramban concludes that the reason is a secret. Perhaps that is why the verse concludes “I am G‑d.” In other words G‑d says that He is the only one who knows the reason for this matter while to humans it is a statute.