In the late 19th century, the first team to translate the entire collection of Jataka tales into English felt this story was so indecent that they published it only in Latin so only scholars could read it. Here is a short version of the story in English.
After the Buddha ended a bitter feud between two of the king’s military officers by preaching about loving-kindness, he told some of his disciples about a past life that he had once shared with those two men.
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The Buddha-to-be once lived as an ascetic deep in the wilderness of the Himalayas. While he sat for his daily meditation, a wicked monkey would come and ejaculate in his ear. Possessing perfect calmness, the Buddha-to-be was not disturbed and did not stop it. One time the monkey saw a turtle laying peacefully in the sun and he stuck his penis in its open mouth. The turtle bit down hard and the monkey was in agony. He lifted up the still-attached turtle and ran to the Buddha-to-be for help.
The Buddha-to-be thought the monkey resembled a brahmin carrying an alms bowl and asked, “Where did you collect so much food?” to mock the monkey’s greed. The monkey pleaded for help and said that if the Buddha-to-be got him free, he would go away forever. So the Buddha-to-be told the turtle, “The marriage between your clans has been consummated, you can stop having sex now.” The two animals released their grips. They both gave respect to the Buddha-to-be and left, the monkey running far, far away.