Do you really know why same-sex marriage wasn't allowed in FFXI? It's probably the exact same thing holding it back from FFXIV, which is to say the Japanese aren't entirely cool with the idea(more like they find it outrageous), and being that FFXIV is a Japanese game, well there you go.

Not to mention that as much as it boggles the mind, same sex marriage is still a red button topic for a lot of societies, and game developers would rather avoid that kind of flak.