A 34-year-old man is admitted to the emergency department after a traumatic landing into a swimming pool from a high diving platform. The patient has multiple traumas in his abdominal cavity. After a reconstructive operation of his abdominal organs the patient develops a high fever. Radiographic examination reveals that the lower portion of the descending colon and rectum has become septic and must be excised. Six months postoperatively the patient complains of impotence. Which of the following structures was most likely injured during the second operation?