In the U.S., we like to think of equality as a good thing. We even have an equal protection clause in the Constitution. And so it is natural to think that all states should create a presumption that after a divorce, the parents should equally share custody of their children.
In some cases, that may be ideal. But not in all. While granting mother's child custody has been a traditional rule in many courts, creating a presumption of equality has consequence. One danger is that it moves the "starting point" in the discussion to one of how to divide the parent's time with their children.