For most of the twentieth century, the courts applied a standard of restrained review to laws involving gender differentiation. They insisted only that lawmakers provide some sort of reasonable explanation for treating men and women differently in the laws that they passed. Most commonly, this "reasonable" explanation drew upon now outdated ideas about women's physical, intellectual, and psychological character. For example, in 1873 the Supreme Court upheld an Illinois law denying women the right to practice law because "…the civil law, as well as nature itself, has always recognized a wide difference in the respective spheres and destinies of man and woman. Man is, or should be, woman's protector and defender. The natural and proper timidity and delicacy which belongs to the female sex evidently unfits it for many of the occupations of civil life. The constitution of the family organization, which is founded in the divine ordinance, as well as in the nature of things, indicates the domestic sphere as that which properly belongs to the domain and functions of womanhood."
In short, a full century after the ratification of the Fourteenth Amendment, the courts were still applying a "restrained review" to laws differentiating between men and women. If the courts could find any "reasonable" explanation for different treatment (no matter how unreasonable we might now consider that explanation today) the law would stand. It took Oklahoma's dual drinking age law to shake, not stir, the Court out of this legal inertia.