The Talmud relates that R. Yoḥanan b. Dahavai said, “The ministering angels told me four things…. Why are people born mute? Because [their fathers] kissed the vagina…. Why are people born blind? Because [their fathers] gazed at the vagina” (Nedarim 20a). However, at the conclusion of the discussion, the Talmud states that this is a solitary opinion, whereas the Sages’ opinion is that these practices are not prohibited (ibid. 20b).
Some Rishonim are stringent, saying that cunnilingus is forbidden, as is gazing at the vagina, though there is no danger involved (Raavad). Other Rishonim say that doing so is permitted, but that it is pious to show concern for potential danger (Smak). Nevertheless, according to the vast majority of Rishonim, R. Yoḥanan b. Dahavai’s opinion is rejected, and the halakha follows the Sages, who maintain that doing so is neither prohibited nor dangerous. Some of those Rishonim even maintain that there is no less holiness in doing so (Yere’im). Many, though, think that while doing so is neither prohibited nor dangerous, a holier and more modest practice is to refrain (Rambam; Smak; Rema, EH 25:2).
In practice, since most Rishonim are lenient, and even those who forbid it agree that the prohibition is rabbinic, it is not prohibited. However, because most Rishonim feel that modesty and holiness make it preferable to be stringent, it is proper to show concern for their opinion (Rema, EH 25:2). However, if one spouse finds it very enjoyable, and their enjoyment will be marred without it, then the joy of the mitzva of ona overrides the stringent opinions, and the couple should follow the majority of the poskim. If they wish to be stringent, they can refrain from this when there is a possibility of conception. If either spouse finds it repulsive, they should follow the stringent view.
There are no limitations on a wife seeing or kissing her husband’s penis. Still, they should not do anything that repulses either one of them. If a specific act is particularly enjoyable for one of them, then even if the other one is not so interested in it, as long as they are not repulsed, there is an element of mitzva in it, for whatever adds to the love and joy of their intimate relations is part of the mitzva of ona as well as the mitzva to “love your fellow as yourself.”
Others maintain that there is no lack of sanctity in this. This opinion is explicit in Sefer Yere’im (cited by Shita Mekubetzet, Nedarim 20b), and can be inferred from Maḥzor Vitri (§528) and Sefer Ha-eshkol (Albeck edition, Hilkhot Tzni’ut 34b), which state that the halakha follows the Sages without making mention of any higher level of modesty or sanctity. Other Rishonim write extensively on the laws and customs of ona without making mention of R. Yoḥanan b. Dahavai’s admonitions. These include Igeret Ha-kodesh (which is attributed to Ramban) and Smag, Lo Ta’aseh 126. It would seem that in their opinion, there is neither prohibition nor any lack of sanctity involved.
Some are stringent and maintain that gazing at the vagina and cunnilingus are forbidden but not dangerous. They write that cunnilingus violates the prohibition of bal teshaktzu (“Do not make yourselves abominable”; Vayikra 11:43). This is the approach of Raavad (Sha’ar Ha-kedusha) and Tur (240; EH 25). It is also implied in Ohel Mo’ed (Sha’ar Isur Ve-heter, derekh 11, netiv 2). SA 240:4, cites Raavad’s strict opinion, as do Beit Shmuel 25:1; Ḥokhmat Adam 128:3; Od Yosef Ḥai, Shoftim §16; Igrot Moshe, YD 2:75; and Darkhei Tahara 22:4.
However, Beit Yosef, EH 25, notes that there is no prohibition according to Rambam, implying agreement with this ruling. This is why R. Karo does not mention any prohibition in SA EH 25. Rema, EH 25:2, writes, “Even though all of these acts are permissible, anyone who sanctifies himself by [refraining from] what is permitted to him is considered holy.” The Aḥaronim explain Rema to mean that it is technically permissible, and it seems that they agree with this (Atzei Arazim; Torot Emet; Ezer Mi-kodesh; Yeshu’ot Yaakov; and R. Kapah’s explanation of Rambam, MT, Laws of Sexual Prohibitions 21:15). Levush 25:2 and AHS ad loc. 11 rule this way as well. The reconciliation of Shulḥan Arukh’s two rulings is that in Oraḥ Ḥayim it states that these actions are forbidden from the perspective of sanctity and modesty; but since they are permitted technically, this issue is not raised in Even Ha-ezer. Others say that cunnilingus and gazing at the vagina are permissible but that there is danger in doing so as part of the intercourse that leads to conception. That is how Kalla Rabbati presents R. Yoḥanan b. Dahavai’s opinion: “All this is if she conceives from this intercourse.” It is also implied by Menorat Ha-ma’or Ha-kadmon, ch. 10.
We can add that perhaps the stringent opinions prohibit kissing the vagina only, not the clitoris, which is the primary erogenous zone. It is also possible that the stringent opinions would apply the prohibition of gazing only to a shaved vagina, since it is then completely exposed. It is also possible that the prohibition of gazing applies only to prolonged ogling in the light, but if it is dark, or the glance is fleeting, it is not prohibited. So states Ezer Mi-kodesh (EH 25:1). R. Yosef Messas (Mayim Ḥayim, vol. 1, p. 92) suggests that the prohibition of gazing is due to the concern that the husband would find it repulsive. Nowadays, though, when there are showers in every home and people wash frequently, there is no prohibition. Perhaps we can explain that since the admonition against cunnilingus is on the grounds of bal teshaktzu, when people wash regularly, there is no concern. All of these uncertainties can be combined with the opinion of the majority of Rishonim that there is no prohibition whatsoever involved.