If they are both Polygamous that function was not supposed to be called in the first place.
What were the exact circumstances of the error?

To repro this issue reliably, you can spawn a bunch of pawns with the psychology Polygamous trait, and pair some of them up as fiances. Once a successful romance check is made between a single pawn and one of the fiances, the game should crash (you can speed this up by modifying the RandomSelectionWeight and SuccessChance to always succeed).

I'm not 100% certain since I'm garbage at code, but I suspect it's because the BreakRelations method runs, the second half of the if/else statement is run, and the nested if statement checks whether one of the pawns is not Polygamous in order to break them up. But because both pawns are Polygamous, this is skipped and the loop starts again, running infinitely.

Sticking a message log in the while loop makes the game print it a bunch of times in the debug log before the game crashes.

Don't be hard on yourself, that debugging analysis suggests a much stronger familiarity with code than you suggest. :)