Leviticus 18:3-6 (the last chapter of Parashat Aharei-Mot) states:

You shall not copy the practices of the land of Egypt where you dwelt, or of the land of Canaan to which I am taking you; not shall you follow their customs. My norms alone shall you observe, and faithfully follow My laws: I am the L-Rd your God. You shall keep My laws and My norms, by the pursuit of which man shall live: I am the L-RD. none of you shall come near anyone of his own flesh to uncover nakedness: I am the L-RD.

When Rabbanim and Darshanim today discuss the question of prohibited sexual relationships, they often mention that whereas the violation of laws concerning prohibited sexual relationships constitutes a grave sin, the consummation of a legitimate sexual relationship in the context of marriage is a great, positive and indeed holy act. It must be noted, however, that in the medieval period, the evaluation of even the permitted sexual relationship (that is, within marriage) was generally not construed in such a thoroughly positive manner. In the interest of seeing what various rishonim said on the matter, it is particularly important to cite the words of Rabbi Moshe ben Nahman (Ramban, 1194-1270). Ramban, in his commentary to Leviticus 18: 6, comments as follows:

The reason for the prohibition of sexual relationships with one near of kin is not expressly written [in the Torah]. The Rabbi [Moshe ben Maimon] wrote in the Moreh Nebukhim (III:49) that [this law seeks to inculcate the lesson that] we should limit sexual intercourse, hold it in contempt, and perform it rarely. Now these women which Scripture has forbidden amongst the relatives of one’s wife are forbidden because they are constantly together with him in the house, and the same applies to one’s own relatives [sisters, aunts, and the wife of one’s uncle], who are frequently with him and he is closeted together with them. A similar reason the Rabbi states for all forbidden relations. Now Rabbi Abraham ibn Ezra has already written (in his commentary to the verse ad loc.) likewise that since the passion of man’s heart is like that of the animals, it was impossible for Scripture to forbid all females, and therefore it prohibited only those that are available to him at all times.

Rambam in the Moreh Nevuhim had also written the following:

[The Sage] says to his son with a view to giving him a rule of conduct: “If you feel sexual excitement and suffer because of it, go to the house of study, read, take part in discussions, put questions, and be asked in your turn, for then this suffering with indubitably pass away.” Marvel at his expression (of Hazal), “this abominable one”, and what an “abomination” this is! This precept is not only valid from the point of view of the Law, for the philosophers consider this matter in the same way. I have already made known to you the text of Aristotle’s saying that this sense is a disgrace to us, he means the sense of touch, which leads us to give preference to eating and sexual intercourse (Guide of the Perplexed [ Pines ed.,], p.608) .

Ramban, for his part, proceeds to dispute the pragmatic reason of the Rambam and ibn Ezra and gives a kabbalistic reason for the sexual prohibitions, connected to “one of the secrets of creation, which attaches to the soul and is part of the secret of the transmigration [of souls]” (Charles B. Chavel, Ramban [Nachmanides]: Commentary on the Torah: Leviticus [New York, 1974], pp. 246-7.)

Ramban’s disagreement with Rambam and ibn Ezra as to the reason for sexual prohibitions does not entail a different evaluation of sexual activity. His comment to Leviticus 18:6 also contains the following declaration:

Know that sexual intercourse is held distant and in contempt in the Torah unless it is for the preservation of the human species, and therefore where there can be no offspring [such as in pederasty or carnal intercourse with beasts] it is forbidden. 

Ramban’s kabbalistic orientation does not in any way imply that he would have a more positive notion of carnal sexual intercourse in general, or of the severity of the sins of prohibited relationships in particular, than either Rambam or ibn Ezra do. Moreover, in the beginning of his commentary to Leviticus, Ramban emphasizes the role that sexual sins of the Israelites had in causing the calamities of their history.

He (God) warned afterwards about those persons with whom it is forbidden to have sexual relations, since association with them defiles, and their sin is called “defilement,” and brings about the removal of the Divine Glory and exile. (Chavel ed., p. 5, and see Leviticus 26:31-32.) According to Ramban, the Israelites are literally thrown out of the land because of such sins. And the only legitimate response is teshuvah.

Ramban interprets Leviticus 26:42 (Then I will remember my covenant with Jacob…) as follows:

I (God) will remember the land, and that it was paid in Sabbaths and lay forsaken without them, and I will remember that they (the children of Israel) have atoned for their iniquity.