It Ended With an Announcement (A Swinging Story) by Kirsten McCurran
The two couples plan a weekend away by a lake, where they can indulge all of their desires. Eve learns to love a new toy and experiences a new first: threesomes. Kurt gets his ultimate fantasy when Eve and Marie join him in bed, and for the first time the Eve and Kurt spend the entire night in bed with their new lovers. No fantasy goes unfulfilled by the end of the weekend, but the couples' relationship is changed forever by an unexpected announcement.