Two backlashes are possible as a result of this decision.
The first, a backlash from opponents of gay rights, is likely to be small. Yes, defenders of Proposition 8 will perform the ritual railing against judicial activists (and complain that the fix was in because the judge who issued the ruling is openly gay). But the facts of the case should put a damper on that. Judge Vaughn Walker is a Republican appointee known for conservative jurisprudence. And the stakes were relatively low; both sides in the case know that this is only one round in a long legal battle that seems headed to the U.S. Supreme Court.
The second possible backlash has to do with California and its direct democracy. Perhaps the spectacle of a federal judge overruling such a momentous electoral result will force Californians to reckon with the fact that their initiative process is at odds with norms of American civil rights and government. At the very least, my state needs routine judicial review of initiatives before they go on the ballot, so the public doesn't face the repeated frustration of passing something only to see it thrown out in court. Maybe this case will produce a backlash that pushes that reform forward.