Let's review some basics.
The courts have consistently held that laws may treat people differently. Even though the Fourteenth Amendment guarantees to all people the "equal protection of the laws," state and federal lawmakers may constitutionally build classifications into the laws that they pass. For instance, nine-year olds may be denied drivers' licenses, and doctors may be subjected to more rigorous licensing requirements than street vendors. To draw the line between equal protection and legitimate classification, the courts have developed a set of tests to evaluate the constitutionality of laws that differentiate between people.
The most widely applied standard is known as the Lindsley test. According to this test, if the classifications created by the law are reasonable and if they are not arbitrarily drawn, they may be compatible with the Constitution. It is not a very difficult test to pass. Using this standard of "restrained review," the courts only demand the existence of some set of facts that "reasonably can be conceived that would sustain" the need to treat people differently. Moreover, the burden of proof generally lies with the person challenging the fairness of the law; as the Supreme Court ruled in Lindsley v. Natural Carbonic Gas Company, "one who assails the classification in such a law must carry the burden of showing that it does not rest upon any reasonable basis, but is essentially arbitrary."
But the courts have also held that certain classifications are inherently suspect, and that laws employing these classifications must be subjected to more "strict scrutiny." All laws that differentiate between people on the basis of race, for example, have been designated by the courts as inherently suspicious and, consequently, the courts demand more elaborate proof that the classifications are necessary to achieve the state's legislative objective. On occasion, the courts also have treated laws that draw distinctions between people on the basis of citizenship as inherently suspect.
The bottom line is that the courts have identified certain types of legal differentiation as more inherently suspicious than others; laws that employ classifications based on race and citizenship are more difficult to legally defend and less likely to survive court scrutiny. But where do laws distinguishing between men and women stand? Are they subject to a "restrained review" or "strict scrutiny"?