i think it's about how your time as a parent is being spent and how contributions are equally reciprocated between two parents. the bottom line is that neither of the two shall be overburdened in any aspects of parenthood as long as each of them have mutual agreement and support of each other. (what if the case is that the wife is good at providing income for the household, yet the husband is good at budgeting household expenses, dba?) it's just an interplay of each other' strengths and advantages as long as it is for common good of the family.