Maybe instead of rings, you could make it something else, since I'm sure there are people who would find it controversial. You could have like....matching boxers/panties. But seriously though, it doesn't have to be rings. It could be gloves, hats, or any number of things. I do think this is a good idea worth exploring though.
I agree with ramen. Maybe you could use some other equips or item instead of ring because of the controversy that it could stir. Plus, that marriage ring in RO could be quite inconvenient sometimes because you have to equip it which means you have to unequip your other accessory).
I know it's some random suggestion but how about some sort of contract/registration between 2 players with an npc as "training partners" with date expiry(the contract goes to your inventory). Special skills will be activated only during the contract period which can only be used with your partner.
Well, i am suggesting a mercenary system and marriage system combined.