Apparently, judging from the whiffs of federalism in his DOMA opinion, Kennedy would have overturned Walker's district court opinion in the Prop. 8 case. But instead of allowing Kennedy to split the baby in half (unlike King Solomon who was merely bluffing), five judges including Roberts and Scalia said the Court had no jurisdiction in the Prop. 8 case. What with the Obamacare decision, Chief Justice Roberts is starting to make a habit of this, or so it seems superficially. We'll see if it develops into a long-term trend. Now that just about every meaningful constitutional limitation on judicial power has been essentially shredded during the past half-century, except for the "case or controversy" requirement, I can understand why Scalia and Roberts would want to defend it, but there is such a thing as over-zealousness.
UPDATE (June 26): On second thought, maybe it's not quite so tragic that the referendum process has been badly harmed today. After all, the U.S. Constitution guarantees a "republican form of government" for each state, and referenda certainly are not republican (small "r"). Additionally, if Roberts and Scalia had unleashed Kennedy in the Prop. 8 case, then Walker's opinion probably would have been overturned but every state would now be weighted down with Anthony Kennedy's marriage code no less than Anthony Kennedy's abortion code. For the time being, states remain free to exercise their own wisdom and judgment on the marriage issue, and Kennedy's animus on the subject has been somewhat restrained.