The traumatic risks of receptive anal sex and large anal insertions include one or more lacerations (which may persist as a chronic anal fissure or anorectal ulcer), hemorrhoidal disease, rectal prolapse, fecal incontinence, and for lengthy insertions, colorectal perforation. Other issues also may arise: inflammation (such as proctitis, or peritonitis following perforation), bacterial infection (and sepsis/abscess/fistula), anal skin tag (remnant of external hemorrhoidal thrombosis, scar tissue e.g. from a healed tear, or a sentinel tag indicating the presence of a chronic fissure), and anatomic stenosis (narrowing due to formation of constricting scar tissue called a stricture). A single instance of trauma can result in development of multiple complications; cumulative damage is a concern as well.
Approximately a few centimeters past the anal opening is the pectinate/dentate line -- the end of the anatomical anal canal, beyond which the lining transitions to the rectum's simple columnar epithelium. Unlike the vagina's durable, multi-layer stratified squamous epithelium, the rectum's lining is very fragile and easily damaged, especially if the layer of mucus normally covering it is removed by an enema. In addition, some enemas and lubricants may cause it to become inflamed or even slough off entirely. Since damage to the rectal lining alone does not normally elicit pain sensations, any rectal problems that develop may remain undetected unless obvious symptoms manifest in the anorectal area and/or elsewhere (such as a fecal bacterial infection leading to an externally-visible fistula).
The anorectal area's physiology also contributes to its fragility. The involuntary internal anal sphincter relaxes with rectal distension, while the external anal sphincter and puborectalis muscles completely relax when a person "bears down." That causes the loosely-attached hemorrhoidal cushions to become engorged with blood, making them more likely to be damaged by pulling force.