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Why did Pope Leo IX oppose the marriage of William I and Matilda of Flanders?

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Engraved portrait of William I, also known as William the Conqueror, shown wearing a crown and medieval robes, facing forward.
William I

At the Council of Reims in October 1049, Pope Leo IX condemned the proposed marriage between William I and Matilda of Flanders on the grounds that it was incestuous, as the two were related in some way. Despite this papal opposition, the couple proceeded with the marriage by 1053. William was eventually reconciled with the papacy in 1059, performing penance by building two monasteries at Caen.1