Why are most courts on a women’s side when it comes to deadbeat parents?

There are fathers that have custody of their children, but when a mother has custody why is the father almost always held 100% financially responsible. For instance if a woman leaves her husband for her boyfriend and takes the children, they live together, neither works, they get on food stamps. The father of the children is held by the state and the court 100% responsible and now owes the state 100% of all state help including all food stamps even though her boyfriend ate the food also. Why is the woman not held responsible for at least half and why doesn’t the state and or the court take into consideration the boyfriends benefiting from the state support. Where are the equal rights? Thoughts?