@squadops
Skimmed Thomas' dissent. It's not particularly good. He always argues from the right starting point, but in this case he doesn't get persuaded far enough away from that starting point to reach the proper conclusion. His starting point is generally "it's not the government's place to decide", and he simply isn't sufficiently persuaded that there is no material distinction between gay and straight marriage to make it an equal protection issue. He argues that there are material differences (and makes an analogy to differences in statutory rape laws that distinguish on the basis of gender), or at least that the lack of material differences isn't properly motivated.
So at least the thought process is roughly correct (as is usual for Thomas) even though the conclusion is botched. He takes a broader view of marriage than simply a legal institution, and that's why he fails to see the lack of difference between gay and straight marriage. I think in this case he'd have been better served to look at it not as a social institution (which is not defined by law anyway), but as a legal institution.