Why did the Prophet, peace and blessings be upon him, object to Ali, may Allah be pleased with him, marrying the Muslim daughter of Abu Jahl, even though Allah permitted marriage to four women and to women from among the People of the Book, and the Quran states, "No bearer of burdens will bear the burden of another," and mentions the presence of enemies and allies of Allah in one household? Was the reason out of fear for Fatimah's feelings or her religious commitment, and what does that imply regarding polygyny?

If you know that harming the Prophet (peace be upon him) is a grave sin, and that Fatima (may Allah be pleased with her) is a part of him, such that what harms her harms him, then your confusion regarding the issue of combining her with the daughter of Abu Jahl will vanish.

The scholars have clarified that the prohibition of Ali combining Fatima with the daughter of Abu Jahl was not a prohibition of what is permissible, but rather for two reasons: one, that it would lead to harming Fatima, which would in turn harm the Prophet (peace be upon him), and whoever harms him perishes; and the other, fear of temptation for her due to jealousy.

It has also been said that the intention was not to prohibit their combination, but rather to inform, by Allah's grace, that they would not be combined.

Others have gone to the possibility that the Prophet (peace be upon him) might have stipulated to Ali that he should not marry another wife while married to her, and this condition is valid.

All of this prohibition is specific to Fatima during her lifetime, out of fear of harming the Messenger of Allah (peace be upon him), or due to such a condition being stipulated by him at the time of the marriage contract.