Some parents are not married. They chose to live together without getting married.
When parents who are not married stop living together, they don't have to get a divorce, because there is no marriage to end. But they do need to decide what will happen to their children and how they will divide their property.
The laws about custody and parenting time or visitation, or who children live with when parents separate, are the same for married couples as they are for parents who are not married.
However, the laws about dividing properthy (the things they own together) are not the same.