First of all, Prop 8 was only passed in California. They can’t overturn Prop 8 in CT because it wasn’t passed here in CT. (Actually, we have gay marriage here, but that’s beside the point.) Prop 8 is a particular law with a particular legislative, political, and judicial history. No other state law against gay marriage shares those same histories. What part of that history becomes the fulcrum upon which the law’s constitutionality is decided will determine how a potential USSC decision applies to other laws.
Secondly, a lot depends on what the question that the USSC actually certifies for review is. The USSC doesn’t necessarily review everything from the very beginning when it grants certoriari. That type of de novo review is rare and limited. Appeals courts, including the USSC, are usually limited to deciding questions of law. The upshot of this is that a decision in Perry v. Schwarzenegger, even by the USSC, might only have immediate impact on Prop 8.
Not really. Let’s say that Prop 8 is struck down on the basis that (to pick something truly silly) the backers didn’t have any representation from Jewish organizations, so they aren’t truly representative of the moral and religious spectrum in California. Then a group in Missouri challenges in Federal Court there a similar law. The Federal District Court will use that precedent to determine if the MO law is constitutional. Let’s say that the MO District Court says: “Yep, no Jews, not a constitutional law.” The higher courts will probably refuse to hear the case, and the law is overturned. No USSC needed.
OTOH, let’s imagine, for the moment (yes, my hypothetical is about to get even sillier) that the MO Federal Court says: “Well, there weren’t any Jewish organizations, but we find that the composition of the proponents is representative of the religious spectrum as it exists in this state, so it stays.” Then that will probably get appealed and the USSC may get involved.
Despite the silliness of the hypothetical, the point is: what effect a potential USSC decision in this particular case has on laws in other states is not clear-cut until the USSC actually takes the case and decides it. A lot depends on what their grounds are for accepting it and what their grounds are for deciding it. It also depends on how closely the now-unconsititutional law is reflected in other state laws. Furthermore, often lower district courts are able to apply a USSC precedent to dispense with the constitutionality of a law in the light of that precedent without further USSC involvement.
Actually, the USSC allows different states to ban some things others call legal all the time. My state bans civilian possession of Tasers that most other states allow. That’s part of what having a federal republic means.
The conservative wing of this Court has especially been immune to “what a legal mess that would create”-type arguments. Scalia and Thomas have both said in different decisions that the practical effects of upholding the law are secondary considerations at best.