Maria essentially makes Freder the chosen one—she inspires him to go underground and gives him his purpose when he awakens to the dystopian system in which he lives. Without her, the story does not proceed and the system continues unopposed.
And, although The Citadel is ruled by powerful men with disabilities, we understand it to be a fundamentally ableist society. Immortan Joe is questing for a “perfect” son and has clearly chosen The Wives for their beautiful, unblemished, able bodies in an attempt to breed one. We understand that this is a patriarchy in its most extreme form where women have no personhood at all.