Whatever is best for the child. Fathers can raise children just as well as mothers!
Custody should be given to the parent who can best take care of the child. The parent's gender shouldn't matter in the decision.
Of course it would be fair! America wants young men to start taking care of their own children and taking responsibility for bringing a child into the world, but it doesn't wanna give fathers custody of the children.
Gender of the parent for custody is far less important than what is best for the child. There is no "one answer fits all" situations.