A castrated man who cannot impregnate a woman is forbidden to marry a Jewish woman (unless she herself is invalid to marry a Jew, for instance, a mamzeret), for it is written, “He who is emasculated by crushing or mutilation shall not enter the assembly of the Lord [marry a Jewish woman]” (Deuteronomy 23:2). The sages ruled that a man who has been “emasculated by crushing” is a man whose testicles have been injured and who therefore cannot fertilize an egg. “Mutilation” refers to one whose sexual organ has been cut and who therefore cannot fertilize an egg. The sages also ruled that if there is a hole in the glans of the penis the man is considered to be mutilated, for he cannot sire children, and so he is forbidden to marry a Jewish woman. One of the sages, Samuel, added that if there is a hole which afterwards is sealed off but when the man had a seminal emission the hole would reopen and widen, he is considered mutilated and as one who cannot sire children. One of the scholars asked: how can we actually check if the hole will open and widen when the man has a seminal emission? Masturbation is, after all, forbidden. One of the sages, Rav Yossef, answered that the man should take warm barley bread and place it on his anus; it will cause sexual stimulation and he will have a seminal emission. Another of the scholars, Abaye, argued that one need no large amount of stimulation to emit semen; it would be enough for the man to see a woman’s pretty clothes. One of the scholars, Rava, asked whether people were so lustful that all it took for them to ejaculate was the sight of a woman’s clothing. Instead, he argued, the suggestion made by Rav Yossef, warm barley bread placed on the anus, was the most practical stimulation for achieving ejaculation.
(Babylonian Talmud, Tractate Yevamot 76a)