Symbolism, Imagery, Allegory
Mary Lou's mother gave her the purity ring to "contain her curse" (14.15)—that being the curse of excessive passion, which she believes affects all the women in her family. It's not stated outright just how the curse works, but it has something to do with the wildness of the girls in her family. Meaning, either they're cursed to be wild, or they're cursed to have bad things happen because of their wildness. In other words, darned if you do, darned if you don't.
When Adina makes fun of the idea of a purity ring, Mary Lou says "Some girls need protection" (4.21). The "from themselves" end of the sentence is implied.
So Mary Lou's ring symbolizes protection, but not the good, loving kind. It's the kind based on fear. If she doesn't embrace her wild side, nothing will happen to her, which is basically how the protection is supposed to work. But let's be real: it's boring to have nothing happen, isn't it?
The morning after she loses her ring, we see a whole new side of Mary Lou. She runs through the jungle naked, "her body strong, her every sense heightened." (14.55) And the journey of self-discovery, finding love, and saving all her friends keeps on chugging from there.
Kinda makes you wonder whether she was more "cursed" with or without the ring.