Role of the anus The anus permits bowel continence, which is voluntary control over the passage of bowel motions. Constipation, straining or pain during bowel movements. Even if you do wash or use an enema beforehand. Digestive system - Rectum and anus Summary We do not know the exact cause of rectal prolapse, but risk factors include chronic constipation, straining to pass bowel motions, and weakened pelvic floor muscles. A University of Minnesota study, assessing the characteristics and risks associated with sexual pursuits of men who have sex with men found that those who met sexual partners both online and offline reported unprotected anal intercourse more often than those who met their partners exclusively online or offline. Thus, aside from those who are really pushing their body's limits, there isn't really any evidence that anal sex is likely to damage the anus.