Let's say that the California Supreme Court doesn't stay its decision -- and I doubt that it would find a legal justification to do so -- but let's say that the California voters reverse that decision in November, by enacting a state constitutional amendment that reads, "Only marriage between a man and a woman is valid or recognized in California." What might happen to same-sex marriages that took place in the months before the amendment becomes effective?
1. One option is that they may remain valid, whether because the initiative is construed as not applying to existing marriages, or because the courts conclude such an interpretation is constitutionally mandated by the Contracts Clause ("No state shall ... pass any ... Law impairing the Obligation of Contracts ....").
I highly doubt that this will happen. According to the text of the amendment, as soon as the amendment takes effect, only male-female marriages are valid or recognized. Future marriages, preexisting marriages, in-state marriages, out-of-state marriages -- all are valid and recognized only so long as they are between a man and a woman. And the Contracts Clause likely won't affect it, since it's been held not to apply to marriage contracts (see Maynard v. Hill, 125 U.S. 190 (1888); Home Bldg. & Loan Ass'n v. Blaisdell, 290 U.S. 398 (1934)), which is why statutes authorizing divorces have been allowed even as to marriages that had been entered into when divorces were not available.
Perhaps the result might be different if there is language in the initiative summary, or the supporters' arguments, that states the initiative will only apply to marriages entered into after the initiative. But I doubt the initiative's backers will make such statements in the ballot pamphlet, or that the state authorities will summarize the initiative this way.
2. Another is that pre-initiative same-sex marriages will become domestic partnerships, which under California statutes give most of the rights of marriage. The proposed initiative doesn't purport to bar such domestic partnerships, and it would make sense to treat such invalidated marriages as domestic partnerships, since this is the result that seems most likely to effectuate as much of the married couples' intentions as possible. In a sense, this would be similar to what courts do when they invalidate legislation on constitutional grounds, including in the same-sex marriage case itself: Since the legislation can't be literally applied, they tend to try to find the solution that the legislature would likely have preferred had it anticipated the court decision.
In the same-sex marriage case, for instance, the court had to implement its equality decision by choosing between treating same-sex marriages as "marriages," and concluding that under state law no marriages could be labeled "marriages." (Recall that even the right-to-marry part of the court's decision left open the possibility that a legislature could simply not use the label "marriage" for any relationship.) The court chose to treat same-sex marriages as marriages, reasoning that "it is readily apparent that extending the designation of marriage to same-sex couples clearly is more consistent with the probable legislative intent than withholding that designation from both opposite-sex couples and same-sex couples in favor of some other, uniform designation." The actual legislative intent of the legislators plus the voters couldn't be perfectly implemented because of the court's constitutional ruling, but the court tried to implement it as closely as possible. One could argue that courts should do the same as to private same-sex marriage decisions invalidated by a state constitutional amendment.
On the other hand, I suppose there might be some same-sex married couples who might take a "marriage or nothing" view, so as to them changing the marriage to a domestic partnership might not reach the result they prefer; maybe there would even be so many that the judgment about what is "more consistent with the probable [individual] intent" becomes unclear. More importantly, there are specific statutory provisions dictating what it takes to create a domestic partnership. A court might well conclude that, unless these formalities are complied with, the domestic partnership can't be said to exist, even if a different set of formalities required for a marriage -- a now-invalidated marriage -- have been complied with.
Given this, I'm not sure how likely a court would be to take this approach; I'd love to hear those who know more about California judicial practices in similar scenarios might be (though note that no scenario has been quite like this one). Note also that the backers of the initiative might well make statements in the ballot pamphlet endorsing this solution -- since such statements might give the initiative more support without deeply offending its advocates -- and those statements might influence the judges deciding how to implement the initiative once it's enacted.
3. A third option is that they will be eliminated altogether, and that married couples will remain domestic partners only if they had entered both into a marriage and into a domestic partnership (on a belt-and-suspenders theory). I wonder, though, whether existing California marriage and domestic partnership law would allow this. My sense is that it should be interpreted to allow this (since this is hardly the same as marrying one person but then becoming domestic partner with another, which is not allowed), but I'm not positive.
4. Finally, it's possible that the legislature will step in, either right after the enactment of the initiative or even before it, specifically providing that any invalidated same-sex marriage will become a domestic partnership. I think that would be good, because it would minimize disruption and best effectuate people's preferences, and I see no reason why it would be unconstitutional. But I'm not sure that it's going to happen.
In any case, just some tentative thoughts -- I'd love to hear yours.