Sayyidah Aishah (may Allah be pleased with her) said: A man used to come in to the wives of the Prophet (peace and blessings be upon him); he was effeminate (mukhannath), and they counted him among "those who have no desire" [Surah al-Nur: 31], that is, impotent. One day the Prophet (peace and blessings be upon him) came in while he was sitting with some of his (peace and blessings be upon him) wives, describing a woman: when she comes forward, she comes with four, and when she turns away, she turns with eight. The Prophet (peace and blessings be upon him) said: "Do I not see that this one knows what is here? This one must not enter upon you." So they screened themselves from him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13550