So, Fritz and his contemporaries being as conversant with Plutarch as they were (I think Fritz read him in translation regularly), I feel pretty confident in saying that's where he got the quote and that he knew the anecdote in question. Whereas if someone today were to use it, I wouldn't necessarily assume they knew the source.

But given how widely applicable the moral of the story is, I don't think Fritz would have limited himself to applying it to marital bed situations. It could just as easily be gambling debts. Though I admit I did think of EC and the imminent move to Rheinsberg when I saw he'd made the allusion to the shoe pinching story!