The only times I brought up marriage was to Oriana, to see how far she and the game would go in terms of relationship, and how it would handle Monogamy/Polygamy. She didn't want to be restrained by things like "marriage", but later forgot or changed her mind after I made a passing comment about "would marry her if she wasn't so against it" and she randomly decided to start calling me husband.
Which leads to the rub. At the moment NPC's memory seems kind of... limited.If you talk to any NPC enough, eventually their memories of your first conversations and even the long term memories you had with them will get flushed out over time with newer stuff. At least it seems that way from the NPC Memories export and gameplay.
In the long run, things like marriage will be forgotten. Unless you remind them with a new long term memory periodically before they forget the first relevant memory. It should be possible that way, but it's a bit too high maintenance to keep track of and curate specific memories for a NPC, much less 4 NPCs.It seems NPC can see what's in your inventory, as they once in a while will point out something I'm wearing or holding. Which has got me thinking... You can get items with odd or weird names if you ask the right people in the right way/narrative situation. Maybe you can use such an item as a form of memory. Like getting a ring named, "(NPC name)'s & (player name)'s Marriage Ring", that way when that NPC scans your inventory, they have something that implies they're married to you every time you talk to them. That's just an idea I came up with.