The basis for a woman's asking her husband's permission to do a voluntary fast comes from the hadith of the Prophet, "A woman should not fast (optional fasts) except with her husband's permission if he is at home (staying with her)." Please reassure your wife that her seeking your permission to perform a voluntary fast is neither demeaning nor degrading to her. Unlike obligatory fasts, which are an obligation solely due to Allah, voluntary fasts involve the rights of a fellow human being, namely the husband. Out of respect and consideration for each other, both husband and wife should check with each other before performing a voluntary fast.
My teacher told me that the fundamental purposes of marriage are love, intimacy, and companionship. It's hard to satisfy these purposes if the spouses do not make themselves sexually available to each other. In Islam, marriage is a form of contract, based on the premise that the woman will make herself sexually available to the man with his promise that he will support her and their children. It is the wife's religious and legal obligation to make herself available to her husband. Likewise, it is the husband's religious and legal obligation to satisfy his wife. Thus, they keep each other chaste.
When a woman gets her husband's consent to perform a voluntary fast, she can attain to much baraka (blessing) because: