The ties that bind… literally. While an engagement ring symbolically binds two people together, Mary's village has a literal Binding Ceremony, in which the betrothed couples are roped together for an evening before the wedding. Aw, how romantic…
Until zombies come along and destroy the village the next morning, that is. So both Harry and Mary, as well as Travis and Cass, are forced to destroy the cords. But that doesn't mean they don't mean anything anymore. Those Binding Cords, or what's left of them, at least, drive Mary bonkers.
Why? Time to tell it like it is (in Mary's words):
With each twist, each tether, each word, I feel myself falling farther from Travis and I must bite my lip to keep from weeping. (13.10)
Turns out the Binding Cords are only romantic when you're in love with your betrothed. For Mary, the Binding Ceremony feels like a steel door slowly swinging shut to lock her into a life she despises. For her, the zombie attack is something of a relief, crazy enough.
I tug at the knots of the Binding rope on my wrist, twisting them, fraying the ends, trying to loosen them. But they hold tight. (16.51)
Even after her escape from the village and the Marriage Ceremony, the cords simply remind Mary of what almost happened. They also remind her that she's trapped in the world of the Unconsecrated—either in the rules of the village or on the paths—until she finds her ocean. She feels like a prisoner in her own world—and let's face it, when you're surrounded by flesh-gobbling monsters on all sides, you are a prisoner.
Suddenly, I feel a profound need to hurt Travis as he stands with his arm around Cass, her fingers possessive around his wrist like a Binding rope. For making me want him so fiercely and for not coming to claim me before my last night with Harry. For not coming before everything grew so complicated and ugly. (19.8)
Mary feels like the Binding ropes, well, bind her to her society by separating her from the things she wants most: freedom and Travis. So seeing Travis with Cass makes her hate him and Cass for allowing themselves to be Bound without love. She knows Cass loves Harry, and Travis loves her, so Cass and Travis's willingness to wear the Binding cords for a night tells Mary that they will allow themselves to give in to the reality of their life in the Forest instead of fighting for freedom. This just doesn't work for her.
I want to run after him, to throw him against the fence and demand him to tell me why he did not come for me before the Binding. I want to blame him for these ropes around my wrist. (21.12)
For all Mary's blame and disgust with Travis and Cass, deep down she knows that she, too, would have given in if the breach hadn't happened and the cords on her wrist remind her of that weakness, so she hates them. Try though she might to place the blame elsewhere, in the end Mary's only got herself to blame… which is all the more reason to keep fighting for the ocean.
Pro tip: If you're being forced into marriage, get out before the zombies force you to. It's cleaner that way.