Leviticus 18:20

Nor unto the side of thy neighbour's wife shalt thou come to defile her.

Some people say that Scripture has already prohibited your fellowman’s wife [Exodus 20:13]; but this specification is needed, to exclude a woman captured in war. The word for seed has been misinterpreted by people whose passions overpowered their reason. We can divide sexuality into three components: 1) procreation, without physical desire; 2) to restore health and physical equilibrium; and 3) sheer animal lust. Scripture uses the word for seed to say that even if your intention is solely to raise progeny, relations with a married woman are still strictly forbidden (the subsequent verse, “Of your seed…” [:21], shows that “seed” means “progeny” in this context). Scripture goes on to say to be defiled through her , for whoever touches a married woman shall not become clean, but shall be forever defiled. This verse does not apply to one’s children through a non-Jewish woman (this is irrelevant to the case of a female war captive, if she converts to Judaism).

AND THOU SHALT NOT LIE CARNALLY WITH THY NEIGHBOR’S WIFE. Rabbi Abraham ibn Ezra commented: “For there is a threefold purpose to sexual intercourse: one is to beget children, a second is to relieve the body of its fluids, and the third is for passion, which is likened to that of the animals. Now when Scripture said l’zara, it means ‘even l’zara’ [i.e., even for the purpose of begetting children], and thus it prohibited [having intercourse with another man’s wife] altogether.” It is possible that He said l’zara in order to mention the reason for the prohibition, since it will not be known to whom “the child” belongs, and as a result, great and wicked abominations might be done by both. Now Scripture did not mention [this expression l’zara further on when it speaks] of the punishment [for this sin],. because even for intimacy of the first stage without emitting seed, he is already liable to punishment. It is for this reason that [in the case of a suspected adulteress] Scripture states, and a man lie with her carnally, literally means “a lying of seed.” for it is on account of the zera that he suspects her. Similarly in the case of a betrothed bondswoman it also says, lieth carnally, which literally means “a lying of seed.” with her, because the prohibition is on account of the child that he will beget from a bondwoman [who is betrothed to a Hebrew servant].
The correct interpretation appears to me to be that since another man’s wife is completely forbidden to one, whosoever toucheth her shall not go unpunished, therefore Scripture had to say here l’zara. For had it said only: “and thou shalt not lie with thy neighbor’s wife,” it would have appeared that it forbids [by punishment of excision] even lying with her just for embracing and kissing, since here [in this section] it speaks only of those forbidden relations that are punishable with excision. Therefore it was necessary to mention that the intimacy was l’zara, in order to explain that He is prohibiting here sexual intercourse. This is also the reason for the expression in the case of the betrothed bondswoman,, which literally means “a lying of seed.” because she is like another man’s wife. Similarly it states in the case of another man’s wife, And the man that committeth adultery with another man’s wife, and it does not state “lieth,” for it is not mere lying with her that He prohibits [under punishment of death]. He does not state here [in the verse before us forbidding intercourse with another man’s wife], “to uncover her nakedness,” for He only mentions this expression with reference to the next of kin and a menstruant, since the prohibition there is because of the uncovering, just as He said, for he hath made naked his near kin.