The Supreme Court will hear oral arguments Tuesday in Little v. Hecox and West Virginia v. B.P.J., which challenge laws that reserve women's and girls' sports divisions in government schools exclusively for females. The great majority of Americans recognize that because men are generally far taller, heavier, faster and stronger than women, co-ed competition is unfair in most sports. But more is at stake than fairness. Those same physical differences mean that allowing male bodies on the court with girls or women is dangerous. We don't allow children to play contact sports with adults because of the obvious physical risk. Similarly, though some variability exists within the male and female pools, the normal range of physical traits differs so widely between the sexes that allowing men to compete in women's sports will inevitably expose women to a heightened risk of physical harm.