This is easy: Race based discrimination is held to a heightened standard. Sex based discrimination is also. However, the majority in this case did not hold that this was sex based discrimination (despite arguments that it was).

That would, in fact, have been valid legal reasoning. Holding that it is sex based discrimination and subject to heightened scrutiny under equal protection would be perfectly reasonable.

Instead, they said that the equal protection clause and the substantive due process clause combine in a magical way to give a new right.