hello thank you for trusting HCM Dear anal sex causes constipation for the exact same reasons that large stools cause it — the diameter of even a small erect penis is as large or larger than the maximum aperture of the anal canal. And if the penis is large and long, it also damages the rectum, which is quite small… And lubrication isn‘t much of a help, because it isn‘t just the friction that does the carnage, but the pressure inside the anal canal that affects anal sphincters, enlarges internal hemorrhoids, tears apart the delicate lining, and damages or desensitizes the super-sensitive nerve endings of the anal plexus.it may cause damage to involuntary contractor muscle weakness may cause constipation. take high fiber diet, regular exercise,avoid anal intercourse. if symptoms not improved please consult your doctor he will examine and treat you accordingly. I think I answered to your question if you have more questions feel free to ask. take care