I believe that the Gois Passage in the story was an important vehicle for the building of the entire story. It was intended to be a romantic setting for remembering good times (per Antoine). However, Melanie's memories were quite different than Antoine's, and this entrance via the Gois set the scene for the whole story.
At the latter part of the book, Antoine's thrilling ride with Angele across the causeway served to emphasize their more liberal thinking that was not shared by Melanie. Antoine just wanted to gift Melanie with the secret, and wasn't able to consider the consequences of revealing it to others in the family. Malanie was very emotional about it all, and wanted to protect her mother's image, and to honor her upbringing by not letting other family members know that she knew what they considered a secret to be kept forever. It was very poignant.