“Im curious. What was that 12th century exception?”
The pope (I believe it was Alexander III in 1169) responded to a request from missionaries in the Baltic region to allow polygamous unions already formed to continue for one generation among recent converts. This was deemed necessary because the splitting up of these unions would have caused a huge humanitarian crisis and potential converts might have put off converting because of affections of the heart and very real needs to care for their children from polygamous unions. The text of the actual letter or decree from Alexander (or whichever pope it was) is in Migne’s Pat. Lat. but I forget which volume (there are 221 volumes in that series so it’s easy to forget). I read it years ago when I was in graduate school. It is an excellent example of the pragmatic nature of missionary work in the Middle Ages.
I knew the "Dark Ages" weren't so dark. I find it interesting that there were still missionary fields in Europe so late in history.