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If the the marriage comes with benefits otherwise not obtainable, it's Level-Up at Intimacy 5.
Slava Ukraini^ wanna ask: would this count as that trope?
- El Sword: you can couple your character with another player's character and then gain a special skill that can heal your HP and MP instantly. Being married will upgrade the skill; the skill will also give you quick mana regen for a short while.
Level-Up at Intimacy 5 isn't what I'm thinking about. I was thinking specifically of the FFXIV "Ceremony of Eternal Bonding" where there's no gameplay benefit but just a way to show that two players who are partners/married IRL can show it in the game (and possibly give Square Enix more money).

Is there a trope for when an MMORPG lets player character get married in the game?