Other than continuing the slow process of convincing Americans of the rightness of their cause, or simply waiting for time to do its work as each new generation becomes more tolerant, these activists have a few options. Attorney Gloria Allred has already announced plans to challenge the constitutionality of the new constitutional amendment - unsurprisingly, this does not seem likely to succeed. Others are hoping to wait a few years and then introduce a new proposition to overturn this one - a prospect which suggests we may see decades of see-sawing court decisions and citizen initiatives.
In the meantime, what will happen to those same-sex couples who have already got married in California? Eugene Volokh has a good rundown of the possibilities. In brief, it appears quite possible that their marriages will be converted to domestic partnerships, either by the courts or by the legislature. Strangely enough, that would not provoke the ire of many who voted for Proposition 8 - a solid majority of Americans are comfortable with something very like gay marriage, so long as it is not called 'marriage'.
Despite the bad news in California, it is worth remembering that Connecticut and Massachusetts still have equal marriage rights, that New York recognises marriages in Massachusetts, and that President-elect Obama has promised to push for federal civil unions. Time remains the best ally supporters of gay marriage have on their side.