One of the five "Inuyim" (modes of self-affliction) that are required on Yom Kippur is abstaining from "Tashmish Ha’mita" – marital relations. It is forbidden for a husband and wife to engage in marital relations on Yom Kippur, and they must also observe all the "Harhakot" restrictions that apply during the time the wife is a Nidda. Thus, for example, they must avoid touching, conversing intimately, and handing objects directly to one another, and they must sleep in separate beds.
Furthermore, Halacha places great importance on avoiding semenal emissions on Yom Kippur, and we therefore observe a number of practices to help avoid such an occurrence. Certain foods that increase the likelihood of nocturnal emissions may not be eaten in the final meal before Yom Kippur, and toward the end of Arbit, before Alenu, we recite a series of chapters of Tehillim that are uniquely suited to help one avoid bodily emissions. The reason why we make such an effort to avoid emissions, as Rashi explains in his commentary to Masechet Yoma, is that it is inappropriate for a person to experience this kind of pleasure while everyone else abstains from this pleasure. Experiencing an emission on Yom Kippur undermines one’s "Inui" and thus, by extension, undermines the atonement achieved through the observance of this day.
The Shulhan Aruch, in discussing these Halachot, writes that a person who experiences an emission on Yom Kippur "must worry the entire year" that perhaps his prayers were not accepted and he was denied atonement. If the individual survives the year, the Shulhan Aruch adds, then he is guaranteed a share in the next world, and will be blessed with offspring and longevity.
Importantly, however, the Steipler Gaon (Rabbi Yaakov Kanievsky, 1899-1985), in a letter published in Karyana De’igarta, writes that this Halacha applies only to exceptionally pious and righteous individuals. Experiencing an emission on Yom Kippur is cause for concern only if there was no natural cause, meaning, the individual was somebody who carefully guarded his eyes and mind and never engaged in inappropriate thoughts that could to lead to emissions. By the same token, the blessing promised to a person who experienced an emission and survived the year also applies only to exceptionally righteous and holy men. An ordinary individual, however, who has not achieved this stature of purity, should not be concerned if he experiences an emission on Yom Kippur. He must, however, earn atonement for his emission; the Steipler Gaon recommends studying Masechet Kelim, which deals with laws of ritual purity. In any event, the warning mentioned by the Shulhan Aruch applies only to holy Sadikim for whom a bodily emission is unnatural and not to be anticipated.
For the rest of us, the concern should be to work toward raising ourselves to this level, and striving to follow the example of sanctity and purity of mind set by these holy Sadikim.
Summary: On Yom Kippur, husbands and wives must abstain from marital relations and must also observe all the restrictions that apply when a wife is a Nidda. Men should avoid eating before the fast foods that may increase the likelihood of a nocturnal emission, and a special series of chapters of Tehillim is recited after Arbit to protect against emissions on the night of Yom Kippur.
