Imam al-Shafi'i (may Allah have mercy on him) said: "Because she (the woman) wants her marriage to him annulled, and (in this case) the oath is upon him (the husband)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / Q14301
Ikrimah (may Allah have mercy on him) said: A woman came to Sayyidah Aishah (may Allah be pleased with her) wearing a green head-covering. She complained about her husband and also showed the mark of a beating on her skin. When the Messenger of Allah (peace and blessings be upon him) came in, Sayyidah Aishah (may Allah be pleased with her) told the Prophet (peace and blessings be upon him) and said: "Muslim women suffer so much from their husbands!" And she said: "There were green bruises on her skin, greener than her head-covering." Ikrimah (may Allah have mercy on him) said: Women support one another. When the man also came, the woman said: "What he has is of no use to me (i.e., he does not fulfil my need)," and taking hold of the edge of her garment she said: "It is like this." The man said: "By Allah, I shake her out the way leather is shaken out, but she intends disobedience, and she wants Rifa'ah," who had divorced her earlier. He (peace and blessings be upon him) said: "If the matter is like this, then it is not lawful for you until you taste his sweetness and he tastes yours."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14301
Uyaynah bin Abd al-Rahman narrates from his father: A woman came to Sayyiduna Samurah bin Jundab (may Allah be pleased with him) and mentioned that her husband was unable to consummate with her. When Sayyiduna Samurah (may Allah be pleased with him) asked the man, he denied it. He wrote to Sayyiduna Mu'awiyah (may Allah be pleased with him) about it, and he wrote back: "Marry him, at the expense of the treasury (bayt al-mal), to a woman who has a share of beauty and religion. If she says that he is able to consummate, keep the two of them together; and if she says that he is not able, separate them." The narrator said: They came to his house, and when people entered I also entered. The man came with traces of yellow on him. He said to him: "What have you done?" He said: "I moved it a great deal in the cloth from behind her." The narrator said: The woman stood by his feet. When he questioned her, she said: "This is nothing." Sayyiduna Samurah (may Allah be pleased with him) said: "Does it not become erect? Does he not come near?" The woman said: "Yes, but whenever he comes near, he brings only harm." Sayyiduna Samurah (may Allah be pleased with him) said: "O you who moves it about, leave her way free." This is the opinion of Sayyiduna Mu'awiyah (may Allah be pleased with him). A man may be impotent with one woman but not with another, and following the Sunnah is better. The Messenger of Allah (peace and blessings be upon him) ruled that the oath is upon the one who denies, and the husband denies the claim of impotence. Amr bin Dinar (may Allah have mercy on him) said: Once he has had intercourse with her one time, there is no further claim or dispute in the matter.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14302