Tafsiran Ulangan 23:1: Rashi, Chizkuni, Ibn Ezra & Rashbam

לא יקח — This does not mean “he shall not take” but “he cannot take” his father’s wife: there can be no question of a legal marriage for him in regard to her, because the marriage ceremony (קדושין) has no legal hold on her (cannot make her his wife: it is no marriage) (Kiddushin 67b; cf. Rashi on Kiddushin 67b s. v. לא יקח)

לא יקח איש את אשת אביו, “a man must not marry a woman who had been the wife of his father;” (even if she had only been raped by his father, and not legally married) This law does not come under the heading of a woman legally married to his father, as the offspring of such a marriage, i.e. the man the Torah speaks about here, would then be a bastard who cannot marry any Jewish woman. If he had done so he would be guilty of the karet penalty, and would forfeit his share in the afterlife. Here we speak of a woman who had been raped by his father, and this is why this verse follows the last verse of the last chapter which dealt with rape, when the penalty had been a financial one payable to the girl’s father. According to the opinion of rabbi Akiva, the karet penalty is sometimes also applicable to violation of commandments where this had not been spelled out. According to the other sages, our paragraph would have to speak of someone sleeping with his aunt while that aunt was awaiting completion of the process of completing the levirate marriage.

A man may not take his father’s wife including a woman who had been raped, but never married (hence the juxtaposition to the previous passage) [the halakha [Yevamot 97a] is that a man may marry a woman with whom his father has had sexual relations outside of marriage — Translator].

ולא יגלה כנף אביו AND HE SHALL NOT UNCOVER HIS FATHER’S SKIRT — This refers to the שומרת יבם of his father (the widow of his father’s brother who died without issue, and who is waiting (שומרת) for her brother-in-law (יבם) either to marry her or to put her through the ceremony of release, חליצה), who is thus destined for his father. But has he not already been prohibited about her (i.e. forbidden to marry her) on account of the law (Leviticus 18:14) “the nakedness of thy father’s brother [thou shalt not uncover]”?! But the prohibition is repeated here in order to make him transgress two negative commands if he takes her (Yevamot 4a), and in order to put into juxtaposition to it the law (v. 2) “one born of incest or adultery (ממזר) shall not come [into the assembly of the Lord]”, and thereby to teach that one is termed ממזר only if he is born from those liable to the penalty of excision on account of the intercourse between them, as is the case with one who take’s his father’s שומרת יבם, who is forbidden to him under the penalty of כרת as אשת אחי אביו; cf. Leviticus 18:14 and Leviticus 18:29 (but not if he was born of a woman intercourse with whom involves only flagellation), and it logically follows that the term applies also to one born from those liable to one of the death penalties by sentence of the court, for amongst the cases of forbidden intercourse there is none punishable with death by the sentence of the court which does not involve the penalty of excision (if it was not preceded by a warning) (Yevamot 49a; cf. also Rashi on Kiddushin 67b s. v. מהנ״מ).