I always thought that Billy Boy was a topical satire about some Prince William or other for whom a marriage or engagement to a very young noblewoman was arranged. (At the same time I also picked up the notions that Georgie Porgie is a topical song about another royal courtship and that Little Jack Horner is a satire about a certain courtier who schemed to secure himself a "plum".) Having read this thread, I postulate that the nursery rhyme as we know it today is a satirical reworking of the Tammy/Willie versions in light of a notorious dynastic marriage.