Years ago, an ex-boyfriend of mine sodomized me accidentally once during sex. My anal sphincter was torn quite badly, and that was after only one thrust of his penis. He was able to get me to the hospital right away, thankfully.
I can't imagine how awful prison rape must be, when you're taking maybe three or four penises in a row. And you probably won't get proper medical attention. And you'll be shitting out semen, possibly infected with various diseases.