A GENERATION or so ago, parents often said, and told themselves, that they were “staying together for the sake of the children.” Where children, and especially small children, were involved, divorce was thought to be clearly harmful. Married couples assumed they had a special obligation to remain together once they had become parents, an obligation that lasted at least until the children were old enough to be on their own. Only extreme conditions could warrant a divorce while the children were still young, at home, and vulnerable.