"for the same reason that Ariosto gives concerning the beauty of Angelica"

   Frankenstein compares Justine's beauty to that of the lovely princess Angelica, who is desired by many noble knights in Ariosto's chivalric romance Orlando Furioso, including the 'furiously' lovesick Orlando himself (See note to page 21).  The comparison is more apt than he knows - at one point in Ariosto's epic the innocent maiden finds herself offered up to a hideous monster as a sacrifice, in place of the selfish villagers who have enraged it.

Online edition of Ariosto's 'Orlando Furioso' as translated and abridged by John Hoole (1791)