A 55-year-old man undergoes low anterior resection with primary anastomosis for a T2N0 rectal cancer located 9 cm from the anal verge. The splenic flexure was mobilized, the inferior mesenteric artery was divided distal to the take-off of the left colic artery, followed by total mesorectal excision. His recovery is uneventful. He returns to the clinic 3 months after surgery describing normal erections, but he is unable to ejaculate. At which step of the operation is nerve damage most likely to have occurred?