Halakhah su Levitico 18:19

To not have intercourse with a menstruant woman: To not have intercourse with a woman when she is menstruant, as it is stated (Leviticus 18:19), "And to a woman in menstrual impurity, you shall not approach." And the time of her menstruation continues for seven days, as it is written (Leviticus 15:19), "seven days shall she be in her menstruation." And whether she [experiences blood] once during them or gushes all seven, she is a menstruant (Niddah 73a). And the entire time that she does not immerse (in a mikveh) - even after the seven - she is [considered] a menstruant; since the verse made it dependent upon days and immersion, as it is stated with impure people (Leviticus 15:18), "and they shall wash in water." And they, may their memory be blessed, said [that it is] a constructive paradigm (binyan av) to any impure one, that he is with his impurity until he immerses (see Rabbenu Bachya on Leviticus 15:19 in the name of the Geonim). And likewise, they, may their memory be blessed, also expounded (Shabbat 64b), "'Shall she be in her menstruation' - she shall be in her menstruation until she goes into water."

He prohibited us from sexual intercourse with a male. And that is His saying, "Any you shall not lie with a male as one lies with a woman" (Leviticus 18:22). And the prohibition about this exact content has already been repeated with His saying, "and there shall not be a male prostitute from the Children of Israel" (Deuteronomy 23:18). And that is the correct approach - that this negative commandment is repeated to strengthen it; and not that it is a prohibition about the one who receives intercourse. Rather, we learn from His saying, "You shall not lie," [both] about the one who lies and the one who is lain with. And in the Gemara, Sanhedrin (Sanhedrin 54b), it is explained that it is Rabbi Yishmael that positions, "and there shall not be a male prostitute," as a prohibition for the one being lain with. Hence one who has sexual intercourse with a male and has a male have sexual intercourse with him in one forgetful spell, is liable for two [sin-offerings] according to the opinion of Rabbi Yishmael. But Rabbi Akiva says, "It is not necessary. Behold, He says, 'And you shall not lie (tishkav) with a male.' Read into it, 'You shall not be lain with (tishakhev).'" Hence one who has sexual intercourse with a male and has a male have sexual intercourse with him in one forgetful spell is only liable for one [sin-offering]. And they said about the reason for this, "You shall not lie and you shall not be lain with are the same." However, "and there shall not be a male prostitute," appears - according to my opinion - to strengthen [it]; like He said, "you shall not commit adultery" (Exodus 20:13), which is the prohibition of a married woman, as we explained, yet afterwards He said, "And to the wife of your kinsman do not give your lying for seed" (Leviticus 18:19). And there are many examples like this, as we explained in the Ninth Principle. And one who transgresses this negative commandment is liable for stoning; and if he is not stoned, he is surely [punished] with excision if he was volitional. But if he was inadvertent, he must sacrifice a fixed sin-offering.

He prohibited approaching one of these forbidden sexual relations - even without intercourse - such as [with] hugging and kissing, and similar such licentious acts. And that is His saying about its prohibition, "Each and every man - to any of his close kin - you shall not approach to uncover nakedness" (Leviticus 18:6) - as if to say, do not make any approach that leads to uncovering nakedness (sexual intercourse). And the language of the Sifra (Sifra, Acharei Mot, Chapter 13:15) is, "'You shall not approach to uncover nakedness' - I only know of nakedness. From where [do we know] not to approach? [Hence] we learn to say, 'To a woman, while in her menstrual impurity, you shall not approach to uncover her nakedness' (Leviticus 18:19). I only know about a menstruant, that she is [forbidden] with, do not approach and with, do not reveal. From where [do we know] about all of the sexual prohibitions, that they are [forbidden] with, do not approach and with, do not reveal? [Hence] we learn to say, 'you shall not approach to uncover.'" And there (Sifra, Acharei Mot, Chapter 13:21), they said, "'Their souls shall be cut off, those who do' (Leviticus 18:29) - what do we learn to say [from this]? Because it is stated, 'You shall not approach,' perhaps they would be liable for excision for approaching. [Hence] we learn to say, 'who do' - and not who approach." And the prohibition about these illusions was already repeated with His saying, "that you shall not do any of the abominable customs" (Leviticus 18:30). However His saying, "After the practice of the Land of Egypt in which you have lived, you shall not do; and the practice of the Land of Canaan, to which I am bringing you, you shall not do," is not only prohibiting the abominable customs - but indeed also prohibiting the actual abominations that He explained after this. And that which He brings two general negative statements about all of the prohibited sexual relations - is because when He prohibited not doing like the practice of the Land of Egypt and the practice of the Land of Canaan, this includes the lewdness, [but it also includes] the work of the land, the shepherding of animals and the settlement of the land as well. So He came back to explain that the actions that He prohibited [originally] were such and such sexual prohibitions that one should not uncover - and it is as He explained at the end of the [section], when He said, "For all of these abominations were done by the people of the land" (Leviticus 18:27). And the language of the [Sifra] (Sifra, Acharei Mot, Section 8:8) is, "Perhaps they should not build houses or plant vineyards like them. [Hence] we learn to say, 'and in their statutes (chukoteichem) you shall not walk' - only those statutes (chukim) which were instituted (chakukim) for them and for their forefathers." And there, they said, "What did they do? A man would wed a man, and a woman, a woman; and a woman would wed two men." Behold it has been made clear that these two negative commandments - being, "After the practice of the Land of Egypt [...] and the practice of the Land of Canaan [...] you shall not do" - are general commandments, being the prohibition of all of the sexual prohibitions (hence they not counted in the tally of the commandments). And afterwards, He repeated the prohibition of each sexual prohibition individually. And we ourselves have explained the regulations of these commandments in the seventh [chapter] of Sanhedrin in our great composition (Commentary on the Mishnah), and we explained that we are lashed for it. And it is from that which is fit for you to know that [in the case of] any woman for whom we would be liable excision, the [child] born from that intercourse is called a mamzer. And it is God, may He be exalted, who called him a mamzer (Deuteronomy 23:3). Whether that intercourse is volitional or whether it was inadvertent, the embryo is a mamzer - except specifically for the menstruant. One born from her is not a mamzer; however it is called the child of a menstruant. And this has already been explained in the fourth [chapter] of Yevamot. (See Parashat Achrei Mot; Mishneh Torah, Forbidden Intercourse.)