Neurotoxin's prediction: Gay marriage wins, unconditionally. The court has been described by the sensationalist media as "deeply divided" in every 'landmark' case in recent years, but it has been on a Federalist binge and there isn't even anyone agruing the States' side this time.
For once, it's nice to have a prediction match an endorsement. Adults' behavior with regard to their bodies or other consenting adults should not be up to a vote, EVER. Marriage as a legal institution carries a wide diaspora of legal protections and the Constitution specifically forbids denying these to any group of people. I absolutely can't comprehend how conservatives who seem to understand the danger of unchecked majority rule in such scenarios most of the time can continue to make a special plea on this issue. This isn't abortion and there's 0 reasonable argument for the other side - much like those who favor gun control, opponents of gay marriage drill down to emotionally charged superstition and a concept they find aesthetically displeasing. No one is asking you to like or accept homosexuals, we're asking you not to bring the government you claim to despise into the mix.