I'm just guessing, but I'm betting they were either trying to make a custom-fit buttplug or really liked the fantasy of being sealed up and figured it'd work loose like superglue does.

Edit: this paper is pure gold, highly recommend looking it up. The photos of the removed epoxy are eyewatering, yet oddly enchanting...

A 27-year-old unemployed young man presented with lower abdominal discomfort following injection of a liquid adhesive per rectum 5 hours previously. He injected the viscous liquids into his rectum via a dual-chambered glue gun, resulting in an instantaneous exothermic reaction that caused the mixture to solidify and become fixed internally. He had no previous history of bowel disorder or psychiatric disturbance. The patient admitted he had done this for anal erotic enjoyment