Although abnormal sexual behavior, including boar-to-boar mounting with anal penetration, is recognized in pubescent pigs, reports of the pathologic consequences are scarce. A 7-month-old male minipig, housed with age-matched males, died within 1 day of the onset of lethargy and reluctance to rise. At necropsy, 2 rectal tears were identified as the cause for fibrinous peritonitis, and spermatozoa were identified in the pelvic and peritoneal cavity by light and transmission electron microscopy. According to DNA typing results, using 11 porcine microsatellites, the intraperitoneal semen was from at least 2 pen mates. The prohibition of castration of fattening pigs, implemented or planned in multiple European countries, could increase the risk of rectal perforation in co-housed pigs.