The SCOTUS dissents with your opinion:
...in the early case of Calder v. Bull(1798), the Supreme Court decided that the phrase, as used in the Constitution, applied only to penal and criminal statutes.
Notwithstanding the fact this is a civil code question, an interesting question is raised. Art I, Section 9's Ex Post Facto clause applies to the Legislative branch of the Federal government, not the Judicial branch since they do not write laws, right?
When the Judicial branch ventures past the separation of powers delineated in the USC into the Legislative powers do such prohibition in Sec 9 apply to the Judicial branch.
SCOTUS may say one thing, but there’s no telling what California’s court will do.
Since Prop 8 is an amendment to the state constitution, it may be overturned only if it conflicts with a higher law. The only law higher is the US Constitution, and in that, the only clause that may be applicable is the Ex Post Facto clause. The California court could declare that voiding the “marriages” legally entered into before Prop 8 passed amounts to a punishment of the persons involved.
I don’t see how the courts could overturn Prop 8 completely, but then again I also don’t see how the 4th Amendment guarantees the right to kill a baby.