Ideas
You could also only upload one of the sims effected by this bug to your gallery. Delete the effectedsim and put the sim you saved to the gallery back in his/her/them family. Use fulleditmode to fix the relationship with the family. In this way you don't loose the familytree If you play a long lagecy. You still have to fix the relationship with the other Sim. To be safe I wouldn't cheat but do it in game, first kiss and all. This way they should be able to woohoo again and continue the legacy.
I used MCCC to reset their relationship but having them do the entire courtship thing was taking way too long. So, I decided to take them back into cas.fulleditmode, only to find I wasn't allowed to change their relationship status. 😮 Oh, come on! Back in game, I cleared their marriage, and that allowed me to make them spouses in CAS again. Phew. I've had to 'fix' this more than once. I'm hoping this third time is the charm.