Unfortunately Eorzea has a polythestic religion with each citystate taking one of the Twelve as their guardian, so unless you worshiped that particular deity you would be out of luck (so Ishgard would forbid anyone getting married there who wasn't already a confirmed member of the Church of Halone for instance). This is why the Sanctum exists, it's for the entire pantheon, not just a single member, which works well for marriages between adventurers who take their own guardians irrespective of citystate. And lore wise it's pretty much unique, being an Amdaporian Fifth Astral Era structure that had been long abandoned only to be revealed in the Seventh Calamity, and restored to it's former glory expressly for the purpose of weddings.

Unfortunately as much as I would like to see weddings in other places like what FFXI had, I think SE are pretty commited to having it just at the Sanctum alas (as it would require entirelly new cutscenes with new building interiors to have to be added first, given weddings are instanced content and all).