Other MMOs such as Final Fantasy online & Elder Scrolls Online both have a marriage system, (although the system in ESO is kinda crap..), These are the only other 2 games I know of that have a marriage system but I'm sure there is a marriage system in more MMO's out there. I know that there is a system in-game as of now where you can confess feelings towards an NPC, I find it odd there isn't one for the players out there.