I don't know if this is of interest as it's so long since the last comments, but I'm just learning a version myself and though I bow to others who have researched the song deeply, the 'magic' idea seems a little contrived. It's an old song and it occurred to me that it might be simply a warning about being promiscuous in the presence of contageous diseases (plague, STD's maybe). The final verse suggests that he was 'putting it about a bit' and the consequences were pretty dire. Just a thought.