1. It is written, "Therefore a man shall leave his father and his mother and shall cling to his wife and they shall be one flesh" (Gen. 2:24). According to the Holy Spirit this verse comes to instruct mankind concerning forbidden relationships. Rashi comments that the phrase, "Therefore a man shall leave his father and his mother," forbids the man to have sexual relations with the wife of his father (not his mother) even after the death of his father, when she is no longer considered a married woman. Obviously, this also includes his natural mother. The phrase, "cling to his wife," comes to teach us that he shall have relations with his own wife and not another man's wife. "To his wife" comes to teach us that he shall have relations with the opposite sex, not with a male. "And they shall be of one flesh" comes to exclude any animal, for an animal is not of one flesh with a man.
2. A Noahide is forbidden to have relationships with certain relatives and others to whom he or she is not related. These are:
Mother (even if his birth had been the result of her having been seduced or raped)
3. Under the Seven Universal Laws, one is permitted to have relations with individuals related through marriage after the death of the relative. According to some opinions, this even includes a man's father's wife (not his mother) after his father dies. Those falling in this category are:
Father's wife (not one's mother)
4. It is the opinion of some authorities that the father's wife is forbidden even after the death of the father, and the prohibition is thought to include women that the father merely had relations with, even if they were seduced or raped by the father. Other authorities permit relations with the father's wife after his death, but forbid relations with either the father's or the mother's maternal sister.
5. It is argued in the name of Rabbi Akiva that all relationships that warrant the death penalty in a Jewish court of law also will receive the death penalty in a Noahide court of law. This includes a relationship that exists through marriage (with a mother‑in‑law or daughter‑in‑law). The reason is that since one's father's wife is forbidden, this is extended to include others related through marriage (a mother‑in‑law being related through marriage). Such relationships are punishable by the courts. Other authorities say that only the father's wife is in this forbidden category, and they exclude other relatives through marriage.
6. Forbidden relationships, other than with relatives, are:
A man with another man's wife
A male with a male
A person (male or female) with an animal
7. A man is forbidden to have relations with another man's wife, whether she is the wife of another Noahide or the Jewish wife of a Jew. Although a Jewish man is forbidden to marry a Noahide woman and from the perspective of the Jewish man there is no marriage, nevertheless, the bond of marriage exists for her and she is forbidden to other men. Under the Seven Universal Commandments, a woman is considered to be a man's wife when the couple has sexual relations with the intent that it constitute marriage.
8. A man is not punished by the courts for having relations with a married woman unless he has sexual intercourse with her in the normal manner (vaginal penetration), inasmuch as this is considered to be the way for a man to be with a woman. But with other forbidden relationships, a man is liable for punishment for sodomy or any other sexual act; it does not have to be the normal way of intercourse.
10. A man is not punished by the courts for having relations with a married woman until she has consummated her marriage with her husband. But if she is engaged and not yet married, even if she is standing under the wedding canopy, and there has been no consummation of the marriage, he is not liable for punishment by the courts. By this law, we are referring to a Noahide man with a Noahide woman. Regarding a Noahide man and an Israelite woman, whether she has consummated the marriage or is engaged and has not yet consummated the marriage or married but has not consummated the marriage, he is liable for capital punishment.
11. In times of slavery, if a Noahide man designated a specific female slave for his male slave and then had relations with her, he was killed because of it. She was considered another man's wife. However, the master was not condemned until it had become public knowledge that those particular slaves had been given to each other. And when did she become permissible again? When she separated from him and uncovered her head in the marketplace (that is, demonstrated publicly that she was available to any man).
12. The concept of divorce with regard to Noahides is a matter of dispute. One opinion holds that there is no divorce possible. Another opinion states that no writ of divorce is necessary, rather that divorce is dependent solely on the volition of either spouse, even if one of them is against the divorce. They separate due to the desire of either one and the thing is done. Others contend that the woman may divorce her husband, but that the husband may not divorce his wife.
13. One who caresses a forbidden member of the opposite sex, or hugs or kisses in a manner of lust, or has close personal contact for the sake of pleasure, transgresses the commandment prohibiting forbidden relationships, but is not punished by the courts. In all cases where the courts are not empowered to act, punishment is meted out by God.
14. It is forbidden to signal with the hands or the feet or to wink at any person who is in the category of a forbidden relationship. One should not be frivolous or light‑headed with anyone in this category, nor should a man deliberately smell the perfume or gaze at the beauty of a woman who is forbidden.
15. A man is guilty of transgressing the commandment forbidding illicit sexual relations by having relations with a male whether the male is an adult or a child, whether the male is consenting, coerced or forced, in public or in the privacy of one's own domicile.
(Note: In the Holy Scriptures, of all the illicit sexual relationships mentioned, only homosexuality is described as an "abomination to God." Even bestiality is not so described. From a spiritual perspective, homosexuality is devastating, destroying both the body and the soul of those who engage in it. No homosexual was ever born into this world through a homosexual relationship. And although it is true that a person may have congenital tendencies towards homosexuality, it is like the child with the trait of tearing out its hair or banging its head against the wall. If the tendency is destructive, the goal is to seek correction. When a person is ill, the goal is to help him get well.
Ultimately, homosexuals themselves will complain against those who misled them by condoning their practices and encouraging them, including the politicians who pander to them as a constituency. In the end they will see that this deviation brings one to excessive, abnormal weakness and to horrible diseases as we are beginning to discover. In the context of history, every society that encouraged or condoned homosexuality was short‑lived, terminated even at the height of its glory. Most notable of these, of course, was ancient Greece, which ruled the world and then was summarily voided in its prime. Ironically, it was the tiny Jewish nation led by the priestly Maccabees that broke the back of the mighty Greek Empire. It is no wonder then that Jewish religionists are so outraged at this society's permissive attitude towards homosexuality, which is the essence of Western society's corrupting Hellenistic legacy.)
16. Though it violates the spirit of the Seven Universal Commandments, lesbianism is not explicitly stated as one of the forbidden relationships. Lesbianism is, however, deemed an immoral and unnatural relationship that destroys the order of the world. Prostitution is in the same general category; that is, while not strictly forbidden, it is outside the realm of morality and therefore violates the spirit of the Seven Universal Laws.
17. Relations with an animal are forbidden at any stage of the animal's maturity, even the day of its birth. A Noahide who has sexual relations with an animal is liable for punishment, but the animal is not killed. In the case of a Jew copulating with an animal, both the person and the animal are killed.
18. The Children of Noah are considered related only through the mother. Those on the father's side are not considered relatives. This means that a man's half‑sister (of the same father but a different mother) is not considered related to him and is permissible to him.
19. There is an argument in the Talmud as to whether a Noahide is permitted to have a relationship with his daughter inasmuch as she is not considered his relative. The conclusion is that, despite the fact that his daughter is not considered to be his relative, she is nonetheless forbidden because she is in the same category as his mother (a parent‑child relationship), and his mother is forbidden.
20. In a homosexual or bestial act, one is liable even if there is only partial penetration.
21. It is forbidden under the Seven Universal Commandments to castrate any male, whether man or animal or fowl. This aspect of the law correlates to both the Laws of Forbidden Relations and the Laws of the Limb of a Living Animal. In one opinion, castration of oneself is a transgression, but it is questionable whether it is a transgression to fulfill someone's request to castrate him or even to agree to castrate an animal of his, despite its clearly being an act of maiming one of God's creatures. Nevertheless, even with so bizarre and irreversible an act as castration, repentance and forgiveness are possible through God's great mercy, as it is written, "For thus has said the Lord concerning the eunuchs that keep My Sabbaths and choose that which pleases Me, and take hold of My covenant. I will give to them within My house and within My walls a place and a name better than sons and daughters; and an everlasting name will I give them that will not be cut off" (Isa. 56:4‑5).
 Gen. 2:24, commentary of Rashi
 Mishneh Torah, Laws of Kings, chapter 9, law 5
 Babylonian Talmud, Sanhedrin 58a
 Mishneh Torah, Laws of Kings, chapter 9, law 6
 Babylonian Talmud, Sanhedrin 58b, commentary of Rashi, "And some learn a mother‑in‑law..."
 Babylonian Talmud, Yebamot 98a, commentary of Ramban (Nachmanides)
 Mishneh Torah, Laws of Kings, chapter 9, law 6
 Babylonian Talmud, Sanhedrin 57b
 Mishneh Torah. Laws of Women, chapter 1, law 1, commentary of the Maggid Mishneh
 Mishneh Torah, Laws of Kings, chapter 9, law 7
 Mishneh Torah, Laws of Kings, chapter 9, law 7, commentary of Kessef Mishneh
 Jerusalem Talmud, Kiddushin, chapter 1, law 1
 Beit Habechira on Sanhedrin, page 227
 Mishneh Torah, Laws of Kings, chapter 9, law 7
 Ibid., law 7
 Babylonian Talmud, Sanhedrin 58b, commentary of Rabbeinu Nissim
 Mishneh Torah, Laws of Kings, chapter 9, law 8
 Bereshit Rabba, chapter 18, opinion of Rav Yochanan
 Mishneh Torah, Laws of Forbidden Relationships, chapter 21, law 1
 Ibid., law 2
 Ibid., Laws of Kings, chapter 9, law 6
 Rights or Ills, adapted from the works of the Lubavitcher Rebbe, Rabbi Menachem M. Schneerson
 Babylonian Talmud, Yebamot 98a, commentary of Rashi, "Behold, the rabbis say..."
 Mishneh Torah, Laws of Forbidden Relationships, chapter 14, law 10
 Babylonian Talmud, Sanhedrin 58b, commentary of Ramban (Nachmanides)
 Ibid., commentary of Beit Habechira
 Jerusalem Talmud, Kiddushin, chapter 1, law 1 and commentaries
 Mishneh Torah, Laws of Kings, chapter 10, law 7, Mishneh l'Melech, "We are returning to our ideas..."
 Shulchan Arukh, Even HaEzer, chapter 5, law 14, see commentary of the RaMoh (Rabbi Moses Isserles)