Midras Yehezkiel 16:32: Midrash Tanchuma, Mekhilta DeRabbi Yishmael, Bamidbar Rabbah & Midrash Tanchuma Buber

(Numb. 5:13, cont.:) “And it is hidden from her husband's eyes,” [is to] exclude one who is blind.³⁹Sifre, Numb. 5:13 (7); Sot. 27a. Cf. Numb. R. 9:10, which explains the exception as referring to a husband just pretending to be blind to his wife’s adultery, so that he can use the rite of the bitter water for killing his wife. Another interpretation (of Numb. 5:13, cont.), “and it is hidden from her husband's eyes.” [These words mean to exclude the case in which] her husband saw [her transgression] and overlooked [it]. (Ibid., cont.:) “So that she is secluded.” We have not yet heard of a specific length of time for her to be in seclusion (with her lover) [in order to cause defilement]. R. Eleazer says, “[The time needed] for a palm tree to sway back.”⁴⁰See TSot. 1:2; Sot. 4a; Sifre, Numb. 5:13 (7). R. Joshua says, “For mixing the cup.” Ben 'Azzay says, “For drinking it.” R. Aqiva says, “For roasting an egg.” R. Judah says, “For eating three eggs one after the other.” R. Eleazar ben Pinhas says, “For a weaver⁴¹Gk.: gerdios. to knot the thread.”⁴²Gk.: nema. R. Hanin says, “For her to put her finger in her mouth.” Pelimo says, “For his (sic) hand⁴³Cf. the parallel in Numb. R. 9:10, which reads, “her hand.” to reach over the basket and take a loaf of bread. And even though there is no evidence for [this understanding], there is a hint [that it is correct], where it is stated (in Prov. 6:26), ‘For on behalf of a woman playing the harlot [one will be reduced] to a loaf of bread.’” (Numb. 5:13, cont.:) “And there is no witness against her.” Although she has no [witness against her] now, she will have one at another time.⁴⁴In addition to Numb. R. 9:10, see Gen. R. 38:14; PRK 18:3; PR 32:2. In a similar usage⁴⁵Kayyotse badavar. This repetitive use of kayyotse b… indicates that the sixth of the seven exegetical rules (middot) attributed to Hillel is being used here. you say (in Gen. 11:30), “Now Sarai was barren; she had no child.” Although [Sarai] had no [child] then, she would have one at another time, as stated (in Gen. 21:1), “Then the Lord visited Sarah […].” In a similar usage you say (in Esth. 2:10), “Esther had not disclosed her people [and her native land].” Although she had not disclosed them to him then, she did disclose them to him at another time, as stated (in Esth. 8:1), “then Mordecai came before the king, for Esther had disclosed what relationship he had to her.” And here also (in Numb. 5:13), “and there is no witness against her.” Although she has no [witness against her] now, she will have one at another time, as stated (in Mal. 3:5), “and I will be a swift witness [against sorcerers, against adulterers].” Our masters have said, “When a woman is alone with her husband and he is having sexual intercourse with her, if she sets her eyes on another at the time of [their] sexual intercourse, there is no greater adultery for her than this.⁴⁶Numb. R. 9:34. Thus it is stated (in Ezek. 16:32), “The adulterous woman [receives strangers] instead of her husband (literally: under her husband).” Is there a woman who commits adultery [while] under her husband?⁴⁷Cf. Numb. 5:19. It is simply that this is [the kind of woman] who encounters a certain man and sets her eyes on him. Then she has sexual intercourse with her husband while her heart is on him. [There is] a story about the king of the Arabs, who asked R. Aqiva, “I am black and my wife is black, but she has borne me a white son. Shall I kill her because she has played the harlot while under me?” He said to him, “Are the images within your house white or black?” He said to him, “White.” He said to him, “When you were busy with her, she set her eyes on the images and bore [a child] like them. Now if you are surprised over this matter, learn from the Jacob's flock. They were conceived from the sticks, as stated (in Gen. 30:39), ‘Since the flock conceived by the sticks, [the flock bore streaked, speckled, and spotted young].’” Then the king of the Arabs thanked R. Aqiva. Thus, when any woman is alone with her husband in holiness, in the end he produces righteous children from her. Thus we find it so in the case of Hannah, who was alone with her husband in holiness, and [so] the Holy One, blessed be He, did not deprive her of her reward. Rather he gave her a righteous son like Moses, as stated (in Jer. 15:1), “Even if Moses and Samuel stood before Me.” It also says (in Ps. 99:6), “Moses and Aaron were among His priests, and Samuel was among those who call upon His name.” So also did Hannah say (in I Sam. 1:27), “For this child did I pray, [and the Lord has granted me my petition].” Why? Because he was sown in holiness. The Holy One, blessed be He, said, “In this world I abhor all those peoples, because they are from unclean seed; but I have chosen you, because you are from true seed, as stated (in Jer. 2:21), ‘And I planted you as a choice vine, all of it from true seed.’ It is also written (in Deut. 7:6), ‘and the Lord your God has chosen you […].’ And also in the future to come I am choosing only you, because you are a holy seed, as stated (in Is. 65:23), ‘They shall not labor in vain, nor bear children in terror, because they are a seed blessed of the Lord, [and their offspring along with them].’”

It is written "There shall not be unto you any other gods in My presence," and, opposite it, "You shall not commit adultery," whereby Scripture apprises us that idolatry is tantamount to adultery. As it is written (Ezekiel 16:32) "You are the (very essence of the) adulterous woman, who (though) living with her husband, (still) takes strangers," and (Hoshea 3:1) "And the L–rd said again to me: "Go and love a woman beloved by her husband, and playing the harlot under him — just as the L–rd loves the children of Israel, while they turn to other gods…"

“The priest shall administer an oath to her, and he shall say to the woman: If a man has not lain with you, and if you did not stray in defilement while married to your husband, be absolved of this water of bitterness that causes curse” (Numbers 5:19).
“The priest shall administer an oath to her” – the priest administers the oath to her, and she does not take the oath on her own. “And he shall say to the woman” – the priest shall teach her the content of the oath; this is the statement of Rabbi Yishmael. Rabbi Yonatan says: If she does not understand his language, he shall say it to her by means of an interpreter.
“If a man has not lain with you” – to include one who performed the initial stage of intercourse from either side.¹⁵¹Front or back. “And if you did not stray in defilement while married to your husband” – to the exclusion of coercion. Just as “while married to your husband” is willingly, here too it is willingly.
“Be absolved of this water of bitterness that causes curse” – he would say to her: If you are pure, drink the water and do not refrain, so that you will become pure to your husband by means of this water. He would administer the oath so that she would drink.
“But you, if you strayed while married to your husband, and if you were defiled, and a man has lain carnally with you, other than your husband” (Numbers 5:20).
“But you” – you did so intentionally. “If you strayed while married to your husband, and if you were defiled” – to include if she was warned regarding one who is nine years and one day old.¹⁵²This is the age at which the Sages determined that a boy is physically capable of engaging in intercourse.
Our Rabbis said: When a woman is in seclusion with her husband and engages in intercourse with him but her heart is devoted to another man whom she saw along the way, you have no greater adultery than that, as it is stated: “Adulterous wife, who takes strangers instead [taḥat] of her husband” (Ezekiel 16:32). Is there a woman who engages in adultery while being with [taḥat] her husband?¹⁵³The word taḥat literally means "under." Rather, this is one who encountered another man and coveted him, and she engages in intercourse with her husband but her heart is devoted to him.
The Arabian king asked Rabbi Akiva: ‘I am a Cushite, and my wife is a Cushite, and she gave birth to a white child; I will kill her because she committed an act of harlotry while married to me.’ He [Rabbi Akiva] said to him: ‘Are the statues and portraits in your house black or white?’ He said to him [Rabbi Akiva]: ‘White.’ He [Rabbi Akiva] said to him: ‘While you were busy with her, she fixed her glance at the white art forms, and she gave birth in their image. If you wonder regarding this matter, derive it from the flock of Jacob our patriarch, as they would conceive in the presence of the rods. As it is stated: “The flocks conceived in the presence of the rods”’ (Genesis 30:39); and the Arabian king conceded to Rabbi Akiva.
Here too, Moses alluded in the Torah: “[If you strayed] while married to your husband, and if you were defiled, and a man has lain carnally with you, other than your husband.”¹⁵⁴Although thinking of another man while being with her husband is called "adultery," the woman is not liable for it and it is not included in the oath. It is not only in this regard¹⁵⁵The man with regard to whom she was warned. that we are administering the oath to you, but regarding what you strayed with anybody other than your husband.
It is taught: “Other than your husband” – to the exclusion of if the other act of intercourse preceded your husband, as if her husband had lain with her afterward, the water does not examine her. But did we not learn: Just as the water examines her, the water examines him. Just as she is forbidden to her husband so she is forbidden to the paramour. Just as she is forbidden to the husband’s brother, so she is forbidden to the paramour’s brother. Just as the water examines her for each and every act of intercourse that she receives from her husband after the paramour, so they examine him. Rabbi Avin said in the name of Rabbi Eila: Here it is where it is known;¹⁵⁶When the husband knew that she had consorted with the paramour. here it is where it is unknown.