I previously opined that the SJC is unlikely to strike down the ban on out-of-staters. But Professor Grossman makes a couple of points that cast doubt on my initial intuition. Most interesting is something I hadn't thought about (but should have): the Privileges and Immunities Clause. Broadly speaking, this forbids states from treating out-of-staters worse than in-staters without a good reason. When fundamental rights are involved, it has to be a very good reason. Is the right to marry fundamental? Pass the popcorn.
I'm still sticking with my prediction that the SJC will uphold the law, but I'm not very sure about it at all.