Sefer HaChinuch ספר החינוך

To not reveal the nakedness of a man’s wife: To not have intercourse with a man’s wife, as it is stated (Exodus 20:13), “You shall not commit adultery.” And the explanation comes that the undifferentiated expression, “adultery,” indicates with a man’s wife, as they, may their memory be blessed, said (Rashi on Exodus 20:13), “Adultery is only with a man’s wife.” And this negative commandment is repeated in the Order of Acharei Mot, as there it is written explicitly (Leviticus 18:20), “And to your neighbor’s wife, etc.”

It is from the roots of this commandment [that it is] so that the world be settled as God desired. And God, blessed be He, wanted that all of His world make their fruit, each and every one according to their species, and not that one species mix with another species. And so [too], did He want that the seed of men be known to whom it is, and not that they be mixed, one with the other. And also, there are several losses found with adultery: That it would be the cause of negating several of the commandments of God to us, as He commanded us in the honor of fathers and they would not be recognized by their children due to adultery; and there would be a stumbling block in that which we were also commanded not to have intercourse with sisters and with many women — and it is all uprooted by reason of adultery, as people will not recognize their relatives. And there is an angle of theft, which is something clear that everybody pushes off. It is also a cause for the destruction of life, as it is very known in the nature of people that they are jealous about the adultery of their spouse with others, and [so] they will fight [to the death] with the adulterer. And there are several other mishaps besides these.

The laws of the commandment — for example, distancing the matter [by] not isolating ourselves with them; and the sentence of the adulterer and also the adulteress, since the adulteress also [has] the prohibition and the penalty; and the rest of its details — are [all] elucidated in Tractate Sanhedrin and [other] places in the Gemara (Mishneh Torah, Laws of Forbidden Intercourse 6). And there in Sanhedrin 51a, it is elucidated that [in the case of] an adulterer with a totally married women, both of them [get] strangulation. And [in the case of] an adulterer with a betrothed maiden, both of them [get] stoning. And with the daughter of a priest, she is burnt and he is strangled. And the prohibition of a man’s wife is from the commandments that are upon all people in the world more generally, whether Jew or whether gentile. But there is a small difference in the matter, as there is no marriage for a gentile except through intercourse; whereas an Israelite acquires her with sanctification (kiddushin) (Sanhedrin 57b).