For the record, I don’t believe the ubiquity of prison rape humor disproves the existence of rape culture, but rather proves it. Zek J. Evets writes at the Good Men Project:
I knew a teenage girl who refused to believe men were capable of being raped by a woman. When I told her the stories about Mary Kay Letourneau, Debra Lafave, and certain Zimbabwean women who even went as far as to steal semen in addition to gang-raping men. She said to me, “that doesn’t count.”
I’ve known grown men who are more likely to believe in UFOs or Bigfoot than some woman who says she was raped. (For the record: UFOs and Bigfoot are real.) They laugh at these women’s stories and slap each others’ backs while calling themselves “good Christian folk”.
Unlike almost any other crime, rape is one in which our private notions of gender, sexuality, and personal responsibility become politicized to the point of oppression. Because of the overwhelming stigmas projected upon rape victims, there are no accurate numbers to describe them. We can only hazard a guess at the sheer amount of Americans who’ve been raped in their lifetime. These uncounted survivors are true subalterns in every sense of the word.
For male victims, it’s like living a double-life, with no resources, no recognition, and no support from the greater whole of American society. Rape-activists regularly discredit men who say they’ve been raped by women, in addition to societal jokes designed to emasculate and shame which results in ultimately silencing these already oppressed men. These factors result in the smallest fraction of male victims coming forward to talk about their experiences, which paradoxically results in greater ignorance supporting the stereotype that men cannot be raped, are not raped, and let’s move on to talking about women shall we? Such actions are ironic among Feminist rape-activists, who passionately protest them in what’s called American “rape culture”.
Read more here.

