I am trying out the engaged couple fighting over how grilled cheese is made scenario and it wouldn't let me choose fiance in the CAS either. I went ahead and chose household member/roommate and the game automatically changed them to engaged once I put them in the world. It's an odd situation to not be able to choose what the scenario expects but glad the game changes it at least. I do think there needs to be a correction or a note about this on the scenario or the CAS page at the very least. Good luck. Hope you can choose household member and get it to work for you.
