הלכה אויף ויקרא 15:16

All who are ritually unclean are bound nevertheless to read the Shema; and, while still unclean, they recite the blessings that precede and succeed it, even if it is possible for them to get rid of their uncleanness, on the same day; as is the case with those who have touched a creeping thing, or touched a woman in the period of menstruation, or touched one who has a running issue, or the couch on which anyone of these has lain, etc. Ezra and his colleagues ordained that only one who had had an emission—as an exception to all unclean classes—may not read the words of the Torah till he has ritually bathed. This ordinance was not adopted universally. The majority of the community were unable to observe it, and it fell into desuetude. All Israelites have accordingly had the custom to read the Torah and recite the Shema, notwithstanding that they had had emissions. For the words of the Torah are insusceptible of defilement, but remain always pure, as it is said, "Is it not thus, my words are like fire, saith the Lord" (Jeremiah 23:29). Even as fire is incapable of being defiled, so the words of the Torah are insusceptible of defilement, but keep their purity forever.

While describing the purification of a man who ejaculated semen, the Torah (Vayikra 15:16) teaches, “He shall immerse all of his flesh in water and remain unclean until the evening.” The Gemara (Eruvin 4b) derives from the words “all of his flesh” that nothing may separate between the person’s flesh and the water.²For analysis of the requirement that all one’s flesh immerse in the water, see Rav Elyakim Krumbein’s essay in Alon Shevut (140-141:92-101). The Gemara limits this problem of separating flesh from water to objects that meet two conditions: they must cover a majority of the body, and the person must object to their presence there (rubo umakpid).³“Objecting” to something’s presence means that one is bothered by its presence while engaging in normal activities, such as working, bathing, or shopping. If, however, one worries about its presence only at the time of tevilah (due to one’s piety), then the object is not a chatzitzah (see Rav Binyomin Forst’s The Laws of Niddah 2:274). It adds, however, that the Rabbis legislated to invalidate a tevilah even when only one of these conditions exists, lest people erroneously permit chatzitzot that meet both conditions. If an object touches only less than half of the body and one does not mind its presence (mi’ut ve’eino makpid), then it does not even constitute a chatzitzah on a rabbinic level since it meets neither condition.

It is prohibited to spill seed needlessly and this sin is more severe than all Torah transgressions. For this reason a man should not thresh inside and sprinkle [his semen] outside [of a woman], and he should not marry a girl who is unable to have children.