(1) This happened sometime afterward: Absalom son of David had a beautiful sister named Tamar, and Amnon son of David became infatuated with her. (2) Amnon was so distraught because of his [half-]sister Tamar that he became sick;... (3) Amnon had a friend named Jonadab, the son of David’s brother Shimah; Jonadab was a very clever man. (4) He asked him, “Why are you so dejected, O prince, morning after morning? Tell me!” Amnon replied, “I am in love with Tamar, the sister of my brother Absalom!” (5) Jonadab said to him, “Lie down in your bed and pretend you are sick. When your father comes to see you, say to him, ‘Let my sister Tamar come and give me something to eat. Let her prepare the food in front of me, so that I may look on, and let her serve it to me.’” (6) Amnon lay down and pretended to be sick. The king came to see him, and Amnon said to the king, “Let my sister Tamar come and prepare a couple of cakes in front of me, and let her bring them to me.” (7) David sent a message to Tamar in the palace, “Please go to the house of your brother Amnon and prepare some food for him.” (8) Tamar went to the house of her brother Amnon, who was in bed. She took dough and kneaded it into cakes in front of him, and cooked the cakes. (9) She took the pan and set out [the cakes], but Amnon refused to eat and ordered everyone to withdraw. After everyone had withdrawn, (10) Amnon said to Tamar, “Bring the food inside and feed me.” Tamar took the cakes she had made and brought them to her brother inside. (11) But when she served them to him, he caught hold of her and said to her, “Come lie with me, sister.” (12) But she said to him, “Don’t, brother. Don’t force me. Such things are not done in Israel! Don’t do such a vile thing! (13) Where will I carry my shame? And you, you will be like any of the scoundrels in Israel! Please, speak to the king; he will not refuse me to you.” (14) But he would not listen to her; he overpowered her and lay with her by force.
(שמואל ב יג, יט) ותקח תמר אפר על ראשה (ואת כתונת) הפסים אשר עליה קרעה תנא משמיה דר' יהושע בן קרחה גדר גדול גדרה תמר באותה שעה אמרו לבנות מלכים כך לבנות הדיוטות על אחת כמה וכמה אם לצנועות כך לפרוצות על אחת כמה וכמה אמר רב יהודה אמר רב באותה שעה גזרו על הייחוד ועל הפנויה
The verse relates that after Amnon raped her: “And Tamar put ashes on her head and rent her garment of many colors that was on her” (II Samuel 13:19). The Sages taught in the name of Rabbi Yehoshua ben Korḥa: Tamar established a great fence at that time by way of her public outcry, as people said: If such an occurrence could happen to the daughters of kings, all the more so could it happen to the daughters of ordinary people. If such an occurrence could happen to modest women like Tamar, who resisted, all the more so could it happen to licentious women. Rav Yehuda says that Rav says: At that time they decreed about seclusion, that a man should not be secluded with women who are forbidden to him, and about a single woman.
(א) איסור יחוד ועם מי אסור להתייחד ובו כ סעיפים:
אסור להתייחד עם ערוה מהעריות בין זקנה בין ילדה שדבר זה גורם לגלות ערוה...
(ו) נשים הרבה עם אנשים הרבה אין חוששין ליחוד
(ט) בית שפתחו פתוח לרשות הרבים אין חשש להתייחד שם עם ערוה:
(1) It is forbidden to be secluded with a person with whom sexual relations are forbidden, whether elderly or young.
(6) If there were many women with many men, we're not concerned about "yichud" [the prohibition of seclusion].
(9) A house whose door is open to the public domain, there is no concern for seclusion there with an "ervah" (person with whom sexual relations are forbidden).