You’ve picked a good topic of discussion, but your question is poorly phrased.
Your claim that only one parent can properly raise children after a marital breakup is false.
Not every divorced parent is capable of exercising joint physical custody or equal physical custody of children.
But when a child has two loving, fit parents who live within the same neighborhood and school district as their child and who are capable of providing personal care and supervision of the child, the evidence is clear that that child will likely do better in the joint or equal physical custody of both parents than if that same child were reared in the sole or primary care of just one of the child’s two parents.
https://fad.lu/wp-content/uploads/2020/08/linda-nielsen-child-custody-60-studies_899.pdf
Articles — National Parents Organization
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