Do you have any Break Up or Divorce options?
It's a pain, but if you have that, you can try breaking off the relationship, then use testingcheatsenabled true to max out the relationship so they aren't enemies anymore, plus remove the various moodlets from breaking up. Then, you could do romantic interactions to put them BACK into a relationship and see if it will let them re-marry.
Save before doing any of that though! I also advise backing up your previous save (load it and do a save as).