When discussing the Prop 8 case, some reference is made to the need for laws to have a "rational basis." It actually is a bit generous to focus on "rational basis" when dealing with gender/sex and fundamental rights such as marriage. At least, as the term is usually defined by the federal courts; some state courts, such as the one involved in the Massachusetts marriage case might be somewhat more strict, even in economic matters.
This is useful to remember because some might find it troubling to submit all laws to some "rational basis" test subject to (often imperfect) judgment of federal judges. As a matter of limiting judicial discretion and focusing on certain important matters, this test is usually very low as to be nearly non-existent if no explicit provision (the Ninth Amendment, notwithstanding) of the Constitution is violation. For good or ill. But various fundamental rights, including those unenumerated (except in general terms, such as "liberty") rights such as marriage and "suspect" classifications like sex/gender* are given "heightened scrutiny," a higher test required.
As to gender:
Without equating gender classifications, for all purposes, to classifications based on race or national origin, [n.6] the Court, in post-Reed decisions, has carefully inspected official action that closes a door or denies opportunity to women (or to men). See J. E. B., 511 U. S., at 152 (Kennedy, J., concurring in judgment) (case law evolving since 1971 "reveal[s] a strong presumption that gender classifications are invalid"). To summarize the Court's current directions for cases of official classification based on gender: Focusing on the differential treatment or denial of opportunity for which relief is sought, the reviewing court must determine whether the proffered justification is "exceedingly persuasive." The burden of justification is demanding and it rests entirely on the State. See Mississippi Univ. for Women, 458 U. S., at 724. The State must show "at least that the [challenged] classification serves `important governmental objectives and that the discriminatory means employed' are `substantially related to the achievement of those objectives.' " Ibid. (quoting Wengler v. Druggists Mutual Ins. Co., 446 U.S. 142, 150 (1980)). The justification must be genuine, not hypothesized or invented post hoc in response to litigation. And it must not rely on overbroad generalizations about the different talents, capacities, or preferences of males and females. See Weinberger v. Wiesenfeld, 420 U.S. 636, 643, 648 (1975); Califano v. Goldfarb, 430 U.S. 199, 223-224 (1977) (Stevens, J., concurring in judgment).
As to marriage: "supported by sufficiently important state interests and is closely tailored to effectuate only those interests."
"Rational basis" is a weaker test:
We have attempted to reconcile the principle with the reality by stating that, if a law neither burdens a fundamental right nor targets a suspect class, we will uphold the legislative classification so long as it bears a rational relation to some legitimate end. See, e.g., Heller v. Doe, 509 U. S. ___, ___ (1993) (slip op., at 6).
["any reasonably conceivable state of facts that could provide a rational basis for the classification"]
The fact that rational basis is not met underlines the weakness of the case. But, the onus is on the state. Some logical rationale or one that has some purpose that is not narrowly tailored is not enough. This is the clear problem of most arguments that even have a shred of logic -- for instance, let's say marriage is deemed important for children, and heterosexuals are more likely to have children by mistake. Marriage serves as an institution that safeguards the results. All the same, this doesn't explain why you have to deny marriage rights to same sex couples, putting aside that they too have children that special favoritism would harm. And, only a limited fraction of married couples would be covered anyways. The state interest is not "narrowly tailored" and is based on "overbroad generalizations."
Any reasonable use of "rational basis" would suggest bans on same sex marriage [you need not be "gay" or "homosexual" to obtain one, so "gay marriage" or "homosexual marriage" appears to me misleading] while different sex marriage are allowed does not hold water. Fantasies about preventing polygamy or question begging appeals to tradition [weakly followed in California, since traditionally domestic partnerships with nearly all the rights of marriage was not in place] aside. The term, however, is sometimes used very loosely, especially when regulating economic matters. So, there is some weak claim of a case there. I address the matter here.
But, since marriage rights and/or sex discrimination (the claim here seems to me largely a backdoor equal protection claim in the first instance -- you have to show that classification by sex is illegitimate in the marriage context) is involved, a stronger case must be met. The law specifically favors a partner of a certain sex. Homosexuality clearly is involved as well but the standard of review there has not been firmly established though even there rational basis cannot merely be met based on tradition or dislike. And, a good case can be made that heightened review is warranted there as in the case of sex.
The lack of rational basis has led the Supreme Court to not need to firmly address this matter in the few cases where it was relevant though some think "rational basis" was applied more strictly here on the down low. Maybe so, but the bottom line holds: marriage and sex discrimination clearly does warrant heightened review, making Prop 8 and other comparable burdens that much harder to defend.
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* It is said that Ruth Bader Ginsburg used "gender" in the 1970s because "sex" discrimination sounded a bit too risque. But, the terms are not really interchangeable. "Sex" is biological, "gender" is cultural. "Masculine" is not the same thing as "male." A butch lesbian is "masculine" but not "male." No matter how much you act like a female, you had the right to vote in 1900, unless there was some other reason you were deprived of the suffrage. The Nineteenth Amendment speaks of "sex" not "gender."
This is important because a large part of this issue rests on what is deemed to be the "natural" activities of each sex, which turns out to be more a matter of societal gender roles. It is possible to talk of "gender discrimination" and know of what you speak, including arbitrary gender roles assigned to a certain sex. But, separating the terms has its benefits too.