Praise be to Allah.
If a husband is fulfilling his Islamic duties towards his wife, it is not permissible for her to ask him for a divorce, because the Prophet (peace and blessings of Allaah be upon him) said: “If any woman asks her husband for a divorce for no compelling reason, she will be deprived of the fragrance of Paradise.”(Reported by Imaam Ahmad, 21874, and Ibn Maajah, 2055). The meaning of the phrase “for no compelling reason” is anything that gives her a strong motive for seeking divorce. (Sharh Ibn Maajah ‘ala al-Sanadi).
As for intercourse in bed, if the wife’s demands are greater than what is normal, it is not permissible for her to seek a divorce (the word “normal” refers to what is usual, such as once a week or once every ten days and so on, and people vary in their capacity in this regard). For more information, please see Question # 1078.
If the husband has some defect or sickness that prevents him from having intercourse (i.e., he is impotent), then it is permissible for his wife to ask for a divorce. And Allaah knows best.