Leviticus 18:6

No man, either young or old, shall come nigh to any of the kindred of his flesh to dishonour (their) nakedness by carnality, or by the knowledge of their nakedness. I am the Lord.

anyone without exception. Since the verse does not mention “of the House of Israel” [e.g., 17:3], we must not allow a resident alien to perform any of these enumerated abominations in our land. He may not pollute our land [see Numbers 35:34].

do not come close a euphemism for sexual intercourse, as in “I came near to the prophetess” [Isaiah 8:3] (compare the related euphemism, “do not approach women” [Exodus 19:15]).

with his near kin This term denotes incest in general. Afterward, its particulars will be listed (HaRav Aaron ben Yosef Sargado HaKohen said that the apparent redundancy of this verse is needed to exclude “wasting of seed” from the incest prohibition).

nakedness something indecent, something which must be covered if it is laid bare.

I am God God loves someone who withdraws from the world in order to serve Him and to understand His words (for example, men and women had to separate from one another three days before the revelation at Sinai). This mystery applied to the very first man [comment on Genesis 1:6] and is intrinsic to the human condition. Since human impulses are animalistic by nature, it was not feasible to forbid all women; but He did forbid all those with whom one is constantly in contact (In the parasha of Ki Teṣeh I shall further reveal a deep and hidden mystery [comment on Deuteronomy 23:10]). The phrase I am God suggests that by dirtying oneself carnally, one distances himself from the Holy and Revealed Name.

NONE OF YOU SHALL APPROACH TO ANY THAT IS NEAR OF KIN TO HIM, TO UNCOVER THEIR NAKEDNESS. The reason for the prohibition of sexual relationships with one’s near of kin is not expressly written [in the Torah]. The Rabbi [Moshe ben Maimon] wrote in the Moreh Nebuchim that [this law seeks to inculcate the lessons 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 his house,, Friedlander’s translation). Now “if we were allowed to marry any of them, and were only precluded from sexual intercourse with them without marriage, most people would constantly become guilty of misconduct with them. But as they are entirely forbidden to us … there is reason to expect that people will not seek it, and will not think of it” (ibid.). 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 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. But this is a very weak reason, that Scripture should make a person liable to the punishment of excision in the case of these forbidden relations, just because they are sometimes found together with him, and at the same time permit a man to marry many women, even in the hundreds and thousands! And what harm would there be if a man would marry only his daughter, just as was permitted to the Noachides, or marry two sisters as did our patriarch Jacob? A person also could not do better than to give his daughter in marriage to his elder son, and they would inherit his possessions and multiply and increase in his house, for He created not the earth a waste, He formed it to be inhabited! We have no tradition as to [the reason of] this prohibition of forbidden relationships, but logically it would appear that there is in this matter one of the secrets of creation, which attaches to the soul and is part of the secret of the transmigration [of souls], to which we have already alluded.
Know that sexual intercourse is held distant and in contempt in the Torah unless it is for the …

איש איש אל כל שאר בשרו לא תקרבו; logic would suppose that offspring from genetically related parents would result in superior human beings, evenly matched ones, as for instance in the case of Amram and Yocheved, where the marital union of aunt and nephew produced three outstanding human beings such as Miriam, Aaron, and Moses. In fact, the Talmud Yevamot 62,63 praises someone who marries his niece by quoting Isaiah 58,9 who calls down a special blessing from heaven on such a person. [“Then when you call, the Lord will answer, when you cry, He will say: ‘Here I am.’”]
The considerations I have just mentioned are valid only if both parties to such a marital union are motivated exclusively by the desire to carry out G’d’s will as they perceive it. However, truth to tell, this occurs in only rare cases. The vast majority of people, when choosing their mate, are driven by the desire to gratify their libido. Compare Psalms 51,7 הן בעוון חוללתי ובחטא יחמתני אמי, “Indeed, I was born with iniquity; with sin my mother conceived me.”
Seeing that genetically close relatives, usually living under the same roof, provide ample opportunity for the males and females of that family to engage in illegitimate sexual relations, and not only do they find it enjoyable but they do not consider it at all sinful, such pairings will hardly ever be for the purpose of marriage, but merely for the purpose of mutual physical indulgence. As a result, allowing marital relations of an incestuous nature would result in sexual promiscuity of major dimensions. This is why the Torah wrote לגלות ערוה, in most instances the very baring of flesh, and especially private parts, is the physical gratification desired in the first instance. This is why the Torah, in order to nip incest in the bud, addresses itself again and again to this phenomenon by using the expression לגלות ערוה, the titillation provided by feasting one’s eyes on the flesh of the opposite sex. When listing degrees of blood relationship, the Torah proceeds from the man’s vantage point, and in the case of the woman from the perspective of her husband. This is why in order of sequence the Torah first forbids the sister of one’s father who is a first degree blood relation to one’s father. On the other hand, the Torah permits the daughter of such father who is a relative of the second degree to the party. The Torah then prohibits the wife of one’s father and the wife of one’s brother and the wife of one’s uncle, even after their respective husbands have already died. This is in spite of the fact that no genetic connection exists between the people forbidden to one another as man and wife. The reason is simply that they Torah views them all from the perspective of how they are related to their husbands. This appears to be the guideline for all incest-related legislation.