The term clearly means engaged, i.e., promised in marriage. Yet, later in the same act we find Hilarion singing about his "baby bride" and their wedding "twenty years ago." The logical explanation is that, although wed as infants, the marriage remained only tentative, and easily annulled, until consummated. There's an unbounded field of speculation on which I could discourse for hours. But, no; instead of that, see what is revealed about Betrothed in The Sorcerer.

To whom our son Hilarion was betrothed
Act I