It’s clear that the Iowa constitution’s equal protection clause, at the time it was adopted, was not understood to prohibit a law limiting marriage to a man and a woman (assuming the issue would even have been intelligible). So the basis for saying that such a law is inconsistent with that clause today has to be an evolving understanding of what equal protection entails, especially regarding what it means to be similarly situated. But barring a constitutional amendment, judges can implement this new understanding only by reinterpreting the clause to mean something it did not mean at the time it was written. That sort of license can lead to all sorts of mischief, as the evolving understanding of the U.S. Constitution’s Commerce Clause (to pick one especially pernicious example) illustrates.
The entire equal protection doctrine that arose out of the transformation of constitutional doctrine in the late 1930’s culminating with the fundamental rights analysis as detailed in Footnote Four of U.S. v Carolene Products is a can of worms. As a result of this ruling, the substantive protections of liberty that were enshrined in due process were applied through the equal protection clause, especially with respect to the judicial review of statutes aimed at discrete and insular minorities. On this note, Sullum is correct about an evolving meaning of equal protection, as the original meaning is that all laws shall be applied equally.
That said, while I recognize how the application of equal protection has gone beyond its meaning, there is textual justification for the Court’s application of heightened scrutiny via a due process or privileges or immunities framework (Section 1 or Section 6 of the Iowa Bill of Rights). Furthermore, the application of the discrete and insular minorities clause of Footnote Four shares important similarities with the traditional state police powers jurisprudence going back to the Founding era, as judges routinely struck down laws that were deemed to be “special interest” or “class-based” legislation aimed at either benefitting or burdening a specific group of individuals (see Howard Gillman’s The Constitution Besieged: The Rise and Demise of Lochner Era Police Powers Jurisprudence for an expansive analysis on this subject).
Furthermore, when Sullum speaks of the “evolving meaning” in this sense, I think he confuses interpretation (linguistic meaning of the text) with construction (application the text in order to create a legal principle that can be reconciled to the document). This should NOT be confused with altering the meaning of the text in the way that, say, liberal courts during the New Deal Era explicitly ignored the Commerce Clause or the Contracts Clause, where the explicit limitations of the text were ignored. Randy Barnett writes (my emphasis added):
In my view, where the (original) meaning of the Constitution is vague, we should select constructions that are consistent with that meaning and that enhance whatever it is that makes a constitution legitimate. Because people operate with different implicit conceptions of legitimacy (e.g. consent, justice, democracy, etc.) there is marked disagreement about how to do construction. Of utmost importance is that, depending on their theory of legitimacy, originalists who are committed to respecting the original public meaning of the Constitution can still differ on how they think construction should be done and, even if they agree about this, they may differ in the wisdom of different constructions. But when these disagreements arise, it is very useful to know what it is we are disagreeing about: not the meaning of the Constitution, but how to put that meaning into effect.
As applied to the Iowa Supreme Court ruling, he application of Footnote Four in terms of the discrete and insular minorities language is a rule of construction (as is, more generally, the extent of the state police power). The underlying legal principle can be both applied to the substantive protections of liberty and reconciled to the text of the Iowa State Constitution (if equal protection makes Sullum uncomfortable, then perhaps the privileges or immunities language of Section 6 or Section 1 in its entirety should suffice). The Court, through a lengthy analysis, determined that a mere rational basis standard was inappropriate given the suspicion that the statute itself was targeted towards a specific class. When the Court analyzed the State’s defense of the statute (i.e. tradition, procreation, etc.) under the intermediate scrutiny standard, the Court rightly found the State’s justifications lacking and struck down the law.
The meaning of the text did not change. An existing legal principle (Footnote Four) that can be easily reconciled to the meaning of the text was applied to a new case and controversy and found that the government had overstepped its bounds. Libertarians should be pleased by this. Not only was justice served, but it was done in a way that kept the meaning of the Iowa State Constitution intact.