There is a very simple rule for which it is hard to find a good reason to have an exception in sentencing law. More serious offenses should have more serious consequences than less serious offenses. Often, figuring out what is more serious and what is less serious is a difficult matter, which we leave to legislative discretion. Balancing the scales is particularly difficult in the case of recidivists who have given long sentences for relatively minor crimes because of their criminal records.
But, sometimes the legislature is simply irrational and unfair. The California case of People v. Hofsheier, decided today, in which a man who had consentual oral sex with a minor woman was required to registered as a sex offender, but would not have had to register as a sex offender had he gone all the way and committed statutory rape via intercourse, is an example of that kind of irrationality. Why? Because, there is no rational basis for calling oral sex more serious than intercourse. Or, at the very least, if there is a rational basis for this distinction, no one arguing on behalf of the state was able to articulate any plausible reason in this case that oral sex was worse than intercourse, despite valiant but far fetched efforts of the state attorney general to the contrary. The seriousness analysis was further born out by the fact that the crime that didn't require sex offender registration carried a longer maximum sentence (four years) than the one that did (three years). Realistically, the legislature probably did what it did in this case because statutory illegal oral sex is buried in a statute full of more serious offenses, while ordinary statutory rape statute section was less complex, and someone likely got slopping in the drafting process for the sex offender registration statute. The California Supreme Court held in a 6-1 decision that this sentence was unconstitutional.
The notable part of this opinion was that the California Supreme Court reached this result not under the 8th Amendment bar on cruel and unusual punishment, which precedent has rendered almost meaningless in these circumstances, but as a result of an application of rational basis review under equal protection clause of the 14th Amendment and also the parallel California provision, which probably insulates the case from U.S. Supreme Court review.
Other examples of a law not meeting this threshold, of which I am aware, are Colorado's Romer v. Evans case in the U.S. Supreme Court, and a case in the U.S. Supreme Court where beer labels were forbidden from displaying alcohol content, while wine and liquor labels were required by law to do so. These cases can alternately be viewed as cases where there is a quasi-suspect category, sexual orientation in the case of Romer and in the case of Limon, discussed below, and free speech in the beer labeling case, as I am aware of no rational basis review cases where a law has not been invalidated in the absence of some sort of quasi-suspect category.
California is itself following in the footsteps of Kansas, which in the case of State v. Limon, 122 P.3d 22 (Kan. 2005), struck down a law that treated consentual homosexual sodomy and consentual heterosexual intercourse very differently in sentencing, and convincingly notes that one of the purposes of the equal protection clause was to eliminate de jure race based sentencing distinctions in the antebellum South. A previous California case, Newland v. Board of Governors (1977), had held that a rule that made the collateral consequences of a misdemeanor sex offense harsher than those a similar felony offense, also failed under a 14th Amendment analysis.
In part, this was a deft sidestep around the unrealistic precedent that a sex offender registration requirement is not a form of punishment. In part, it was a silent recognition of how crabbed 8th Amendment precedents have grown. Indeed, one could almost describe Limon and this case together as creating a quasi-suspect category in the area of sentencing that raises 8th Amendment issues without actually satisfying the requirements of the 8th Amendment.
In contrast, the U.S. Supreme Court decision in Lawrence v. Texas, in which it struck down laws banning homosexual sodomy between consenting adults, rested on the due process clause, arguing that there is a substantive protection for consentual adult intimate sexual activity.
Admittedly, the situation addressed by this particular case is narrow and quirky. But, to the extent that the constitutional precedent that one offense may not have a sentence more harsh and another indubitably less serious or equally serious offense is established, one can imagine a significant number of cases that more obviously seem like cruel and unusual punishment cases being resolved justly using the equal protection clause of the 14th Amendment. Essentially, these precedents have the ability to make to "cruel and unusual" into "cruel or unusual" which probably better reflects the original intent of the 8th Amendment. Given that the state level application of the 8th Amendment flows from the 14th Amendment given current constitutional incorporation doctrine in any case, there is even good reason to favor the 14th Amendment analysis which is more direct than the incorporation analysis, and thus less prone to the vaguries of decisions by some future conservative U.S. Supreme Court, than the 8th Amendment analysis, whenever possible.