Roberts had an interesting argument in his dissent, but he didn't really explain why the 14th amendment doesn't necessitate equal treatment via marriage. (The Equal Protection clause has already been incorporated in individual states in cases before, so yes it applies I think)
So as far as I can see there isn't a significant legal argument against this. I think had the Supreme Court not done this, it would have inevitably become legalized in all the states anyway, (maybe the South would take a while, as they always do) but it's cool that it came so quickly.