I'm not a journalist, and I'm about to present proof in case there was any doubt. But it so happens that I'm from a small town that happens to house a maximum security prison and a hospital, and I know any number of people who have or do work there, as nurses, guards and others.
After Spoons' post noted below, I got morbidly curious because I didn't recall ever hearing anything about prison rape from the local prison. This could be because it was unheard of, or because it was so commonplace that it wasn't remarkable. I thought I'd find out.
Have you ever cold-called someone and asked them about prison rape? That poor woman is probably still creeped out. You see, I called the local hospital and asked if they ever got inmates delivered there for uh, reconstructive surgery after such an event (I had heard that they had). The woman at the emergency room stood up well under the coyly phrased questions and noted that they didn't see much if any of that or other prison-related injury. She also noted that the prison had its own health facilities. She was amazingly game, but she clearly wanted off the phone, and it was an emergency room after all. Remind me to work on my interviewing.
Next stop - call some people I knew from town about the topic, whether for scuttlebutt or direct news. Nobody recalled ever hearing anything about such events, in a town with plenty of prison guards who hang out in local bars. Hmm, interesting.
Next - call a person who I know very well, who had worked in nursing at the prison, who no longer works there, and who has no reason to shade the truth. How often was prison rape seen? She didn't recall any that required the attention of her staff, over a three year period. She noted that prison sex was against the rules, so there was an incentive to cry rape. And over her tenure, they had only seen two instances of medical conditions that implied inmate-inmate sex of any kind.
OTOH inmates were ingenious at getting it on with visitors. Wearing big coats provided cover for someone else within. They could gather around picnic tables as a screen while taking turns screwing beneath. I've heard that woman prisoners have managed to get pregnant while serving time, and not by the guards either, but I can't find a link as this is written.
Caveats about the above: obviously it's anecdotal. The prison in question is maximum security, so inmates are more constrained in their movements than they are at other facilities. And different states do things differently. But we have to start somewhere, and you're getting every bit of what you pay for.
Here is what Human Rights Watch has to say. The short answer is that they don't know, and attempts to answer are all over the map. Extrapolations are treacherous because practices vary from state to state.
And HRW is biased, as evidence by this: "Unsurprisingly, when corrections officials are asked about the prevalence of rape in their prisons, they claim it is a exceptional occurrence rather than a systemic problem." Why is this unsurprising? Obviously they presume that corrections officials would lie about this. Certainly they might, but then so might the prisoners and advocates who provide the more spectacular numbers.
My personal conclusion is that activists are magnifying a problem, as usual. Prisoners and lawyers are always looking for grounds for lawsuits. And I don't hear the activists proposing sanctions against the prisoners who are doing this - it's always the prison that needs to change.