what does "Borderland of life's mystery" symbolize?

This quote shows an example of sex being made enjoyable “When he touched her breasts they gave them selves up in quivering ecstasy, inviting his lips. Her mouth was a fountain of delight. And when he possessed her, they seemed to swoon together at the very borderland of life’s mystery” (Chopin 3-4). The affair that takes place during the storm, between two married people, does not ruin either of their marriages, but instead strengthens both of them.