So these female NPC's say that I am already married, I started a new world (MC 1.12.2) in creative mode,
I get the relationship to 5 gold stars and when I give them the engagement ring, they say "You're already married"

What the f**k ?

Please look through the list of issues before reporting, there's probably eight or ten reports of this bug up already. Briefly, you start off being married to "?", that is a nonexistent person. If you get a cleric to give you a divorce, you can then get engaged and marry someone, but when you log out/save & exit, your marriage will reset to ?.

So just to be clear (sorry for re-opening), the MCA wiki said you needed a priest. Which one is needed, a MCA priest or a vanilla cleric villager?