If California passes a law (and it has) saying that registered domestic partners may inherit from each other even if the two same sex partners do not have a will, and someone challenges the inheritance in court by claiming that California law does not allow domestic partners to inherit from each other, what would the proposed Constitutional amendment require the court to do?
Can the court construe California law to allow inheritance without a will, which is plainly the intent of the California Legislature? Isn't inheritance without a will by persons who are not related by blood a legal incident of marriage?
My take would be that only those state civil union laws that make an implicit equation with marriage law would be voided if and when FMA is ratified. If the civil union law explicitly makes an accomodation, then it is no longer a legal incident of marriage in that instance.
This creates a bit of a mess, though. A parallel set of laws. A parallel set of judicial precedents. Judges have a nasty habit of creating doctrines to eliminate untidiness.