First, per SCOTUSBlog, and a hat tip to Perry, I think that a 6-3 decision in favor is possible. Likely? Not as likely as a 5-4 maybe, but more likely than a 5-4 against.
A "tell," on the current reporting, is Justice Kennedy's higher emotional level when asking questions of the plaintiffs (the states where an appellate court said gay marriage should remain illegal) about whether this wasn't disruptive to gay couples with children and other things. (At the same time, this is why Kennedy tilts pro-life, yet I think would oppose any attempt at an outright ban of abortion.)
Otherwise, I think Kennedy, in raising the past history of marriage, was trying to encourage the defendants, gay marriage supporters, to address that issue as part of their narrative. His later questioning showed that what he has elsewhere mentioned as the "dignity" and "sacredness" of marriage is a key issue to him, not the argument for biological procreation.
Meanwhile, in additional current reporting, Nino Scalia must be slipping. In his interrogations today, he never once compared gay marriage to bestiality. Justice Alito at least partially picked up for him by raising polygamy as a red herring.
(Actually, as long as the person already in a marriage informs a potential second or third spouse about said other marriage, I'm OK with that. Make it felony fraud not to do so, and we go from there. Besides, Religious Right, let's not forget that the "Judeo" fig leaf half of your "Judeo-Christian values" was so "down" with polygamy that Jesus' ancestor Jacob married multiple wives, as well as having additional kids by legally recognized concubines. Beyond that, the Judeo was so down with other open marriage ideas that Abraham married his half sister. Good times!)
Roberts, per both those top links above, could indeed "swing," so to speak. Per Perry, he fought against anti-gay discrimination in the workforce and won, on the major Romer case. Per the arguments now, Roberts at a minimum seems to give the Constitution's comity clause — that states should respect each others' laws — a fair amount of weight. If Texas is forced to recognize gay marriages from Illinois, then it's essentially game over anyway.
And, speaking of Texas, our lord high executioners in the Legislature are already seeking to craft various workarounds, at the state level, should the Supremes legalize gay marriage.
Of course, the justices' ruling isn't expected until after the Lege's session ends.
If wingnuts led by Rick Perry called an anti-abortion special session in 2013, you bet wingnuts led by Greg Abbott will consider the same this year. Bank on it. And lawsuits against the feds and more.
After all, Gov. Strangeabbott, despite his alleged fiscal conservative credentials, is our state's chief money waster.