There's a blog post at This Ain't Livin' called The Myth of Cushy Prisons that is well-worth reading. It focuses on the material conditions of the prisons themselves, but the way prisons are run is worth bringing up as well, because in that area the myth goes from being merely mistaken to downright nonsensical.
On the contrary, open acknowledgments of it are pervasive. Prison rape is not an official part of the legal system, but it's so common and so tolerated that it might as well be. References to it, both serious and comedic, are ubiquitous in pop culture; referencing it is risque, but hardly shocking or taboo. Protagonists on prime time network cop shows threaten uncooperative men with it, and this is generally not considered shocking or unheroic because everyone understands- even if they do not say- that being raped in prison is a de facto component of many prison sentences.
Now, it's true that this sort of incoherence is not unique in political matters. As Roderick Long has pointed out, modern statism in general depends on people's belief that the state is a peaceful, consensual institution and their knowledge that it actually isn't, existing side-by-side. But while the reality of the nature of the state is obscured by a veil of ideological obfuscations, that's not the case here. People may try to rationalize or justify or condone the prevalence of violence in prisons, but rarely if ever try to claim it's not violent, or isn't horrible for the victim.
It's also true that people trying to deny an intolerable reality can develop irrational, absurd, or blatantly and obviously self-contradictory beliefs to keep themselves going. But this isn't about denial- most people know about it and will acknowledge it if the subject comes up, and some outright revel in it. The subject is not taboo. People may not know the precise details of how prevalent it is, but it's widely understood that it is not a rare, unusual occurrence happening in a generally peaceful and safe environment.
If anything, there seems to be a positive correlation between openly acknowledging what prisons are like and the stated belief that prisons are "country clubs"; my own experience is that people who lament the overly luxurious conditions of the American prison system are more likely than average to openly chortle at the prospect of someone they dislike being raped in prison. Somehow, they're able to reconcile the two. Country clubs are less genteel than popular stereotypes have led me to believe, apparently.