Is it permissible for a wife to refuse her husband's advances due to his sexual weakness and inability to satisfy her and keep her chaste, and is she sinful for doing so, and what is the ruling on a husband who does not help his wife maintain her chastity?

A husband is obligated to ensure his wife's chastity (or sexual fulfillment). His refusal to seek treatment or his claim that this is the best he can do does not absolve him of responsibility if he fails to fulfill her needs. The wife has the right to seek separation, such as divorce (talaq) or annulment (khul'). It is not permissible to imagine forbidden things to satisfy one's urges. Furthermore, it is not permissible for a wife to refuse her husband's bed on the pretext of avoiding sin; rather, she must be patient and seek divorce if she is harmed.