The feature has only been barely friggen mentioned.
How about we just have a little patience before asking how things will work? I mean, are you expecting players to answer you? Community reps (with the content not even having a full blown announcement yet)???
The only information we have is that weddings will likely take place at the Sanctum of the Twelve, the wedding veil is gonna be wavy and pretty- and some old text from Yoshi stating that same sex might result in "backlash" so they would likely only offer opposite-sex marriage at the start.

We have NO clue how Phantasia will affect married couples. NO clue if there are/aren't benefits of marriage (though shared storage would be awesome ).

TL;DR- Patience. We will know all we need to WHEN we need to.