I can envision a different outcome. If totally defeated by the US Supreme Court the Republican Party would press for a US Constitutional Amendment to outlaw gay marriage and unions. Such an amendment would become part of the Constitution, superseding any prior court rulings, and preventing any future cases to allow gay marriage.
The ratification process would be fought at the State level, something Republicans in the past have been very good doing in opposing gay rights. The first obstacle would be getting the Senate and House to send the Amendment to the States, not possible with the current Congress (the President plays no part in the process, and has no veto).
But the 2014 Congressional election could change that balance to give Republicans the votes they need to begin the amendment process, and they could very well make the anti-gay amendment a major issue for their candidates in State Congressional races.
If Prop 8 is upheld, however, but DOMA gets overturned, that means individual State could ban gay marriage, but States must recognize gay marriage from those States that do allow it. The Republican goal would then be to outlaw gay marriage in the States that now permit it, as they've attempted before, so that the repeal of DOMA would be meaningless if no States actually had gay marriage to recognize at the Federal level.
I see the Republicans as too committed to their core constituency to retreat on their stance against gay marriage & unions, or on any gay civil rights. But since public opinion is changing on this issue, the Republicans may be drawn like moths to their ideological flame, and be unable to resist their own political decline.