My guess is that the Supreme Court rules that while a federal ban on same-sex marriage is unconstitutional, a state-wide ban is constitutional and ruling that the constitution does not give federal courts the authority to strike down state-level bans on same-sex marriage. I doubt Kennedy would have any problem signing off on such an opinion. In cases like Bush v. Gore*, D.C. v. Heller*, Shelby County v. Holder, Citizens United v. Federal Elections Commission, Sebelius v. Hobby Lobby, etc the five conservative justices have proven that they will pay the Constitution no more mind than toilet paper when it conflicts with their personal political agendas. The question isn't whether the Supreme Court's conservative majority will once again violate their oaths of office, but merely how far they will go. Even if they rule that statewide bans are unconstitutional, it will only be because the Republican Party believes it is in its political interest to put this issue to bed (I suppose that would be another "who cares why people do good things" situation as Al put it in another thread).
*Alito and Roberts weren't on the Court at the time of the two * rulings.
that is just paranoia. You may not always agree with Justice Kennedy, but he isn't some partisan hack who is taking orders from Reince Priebus. And he isn't one of those 'states rights' types of guys either. Kennedy also wrote the majority opinions is Romer v. Evans (overturning a CO ban on any laws protecting gays from discrimination) and Lawrence v. Texas (which overturned sodomy laws).
Here is part of his opinion from Romer (way back in 1996)
In a way, he really isn't a swing vote in this case. He has made it clear that he sees sexual orientation as protected by the fourteenth amendment just like race. When you look at his history, it seems impossible that he will suddenly back SSM bans, especially for partisan reasons.