The early sages (the Tanaaim) ruled that a woman must examine herself by using a piece of cloth in her sexual organ, lest there be drops of blood and she be impure. There is no fear that feeling within her sexual organ will lead to sexual stimulation. But men are not to check themselves lest semen be ejaculated from their sexual organs, for there is a likelihood that due to the examination they will feel their sexual organs and sexual stimulate themselves, leading to an unnecessary seminal emission. If the man does wish to examine himself he should not feel his sexual organ with his hand, but use a hard object like pottery or a stone, for the touch of the stone to his sexual organ will not cause sexual stimulation. One of the scholars, Abaye, permitted examining with a thick cloth which, being hard, does not cause stimulation.
The sages also ruled that a priest who is eating terumah [which must be eaten in purity, without a previous seminal emission] and feels so strong a sexual stimulation that he begins to ejaculate must hold his sexual organ and stop the ejaculate so he will not become impure while eating terumah. One of the scholars, Abaye, rules the one is only permitted to stop the ejaculate using a thick cloth, to avoiding adding stimulation to the already-present stimulation. According to a different scholar, Rava, if the semen has already left its point of origin, though it has yet to be ejaculated outside of the body, one may hold the sexual organ even with a soft cloth; since the semen has already moved there is no fear that additional stimulation will cause an increase in the ejaculate. The scholars asked: Did not the early sages compare this situation to one who pokes a finger into an eye — as long as the finger remains in the eye the eye continues to tear”? Even if the semen has begun to exit, there is the possibility of ejaculating more semen through stimulation. Answer: It is not common that a person will become additionally stimulated.
One of the early sages, Rabbi Eliezer, ruled that a man who wishes to urinate should not hold his sexual organ and urinate, lest his holding cause sexual stimulation and he ejaculate unnecessarily. The sages asked him: If one urinates and does not hold is sexual organ, drops of urine fall on his pants. Seeing these drops, people will conclude that he had a seminal emission due to an illness which damaged his testes or urethra and will then think that his sons are not his own, for a man with such damage cannot sire children, for they have seminal emissions without erection. Rabbi Eliezer answered: It is preferable that people should slander a person and think his sons to be bastards than that he should hold his sexual organ in his hand and cause a seminal emission, for any who has an unnecessary seminal emission is liable to the death penalty. An incident involving two sages, Rav Judah and Samuel, who stood on the roof of the synagogue: Rav Judah told Samuel that he had to urinate. Samuel told him to hold his sexual organ and urinate so drops of urine would not fall on the roof of the synagogue. The scholars asked: How could Samuel give such advice, suggesting the other man hold his sexual organ and urinate? Answer: Since Rav Judah was standing on a roof, due to fear and awe he would not become sexually stimulated. Another answer: Because of his great awe in standing next to Samuel, he would not become sexually stimulated. Yet another answer: Since he was urinating off the synagogue roof he would feel awe of the Shechinah, which is present in the synagogue, and would not become sexually stimulated. Another answer: Rav Judah was married and married men, who have a woman to satisfy their lusts, need not fear sexual stimulation. Yet another answer: Rav Judah held his testicles from below and not the actual sexual organ; holding the testicles does not lead to sexual stimulation. Another answer: Rav Judah held his glans and urinated; one who holds his glans is not sexually stimulated as long as he does not hold the actual sexual organ.
(Babylonian Talmud, Tractate Niddah 13a-b)