Is a girl sinful for refusing to marry a man who is already married to another, due to her intense jealousy, her fear of what might befall his first wife, and her desire not to do to others what she would not wish done to herself, even while acknowledging the permissibility of polygyny in Islamic law?

A woman does not sin by refusing to be a second wife to a married man. She may also stipulate to her husband that he not marry another woman, and this does not mean she is prohibiting polygyny. A woman's jealousy is natural, but it should not prevent her from agreeing to polygyny, due to the noble interests and objectives for which Islam legislated it. Therefore, a woman's refusal of polygyny under the pretext of "leaving something for the sake of Allah" or "what I do not wish for myself, I do not wish for others" is a misguided application of these concepts. She should agree to it seeking Allah's pleasure to be rewarded. It is unjust for society to blame a woman who accepts to be a second wife.