The Supremes will decide marriage is not a civil right and is not a federal issue. They will kick it back to the States.
That's a distinct possibility. They've been known to punt in the past.
If they do, here's what you'll probably see: a endless succession of 'fence-sitting' states doing what others have done, over the next 3-5 years. By 2020, there will be but a half-dozen hold-out states that still officially oppose the change, and the rest of America will have moved on.... and the 'quality of life' for almost all Americans won't have been diminished one whit. On the other hand, the quality of life for a significant number of American citizens will have improved substantially.
I say this with no malice, glee, sorrow or angst whatsoever. I simply see the momentum of this movement as part of the overall subject of 'inexorable change.' It isn't good OR bad. It simply is
the way of things. Such has it always been, since time immemorial.
I don't really know how The Supremes will fall on this issue. It's still not a foregone conclusion either way at this point.
MY GUESS is that they'll change the law to legitimize gay marriage, 5-4. Roberts will (again) cast the deciding vote, and his vote will be motivated as much by 'optics' and legacy, as he'll be motivated by the letter/spirit of Law. Face it- he's as much a 'politician' as any House or Senate player... and even HE can see that it's only a matter of time. He'll choose yea, rather than see someone else's name get the credit for something that is already inevitable.
...but that's only my guess.