I used to work as a nurse in a prison in Pennsylvania. During physicals and behavioral assessments I would be able to tell if someone was being sexually abused. Examined thousands of prisoners and I never saw any evidence of rape. I'm not saying it doesn't exist but I am saying it seems rare. More than enough consensual sex is happening and open homosexuality either of the top or bottom variety isn't respected at all.

Edit: From colleagues I've heard of more stories of rape and abuse by male correctional officers of female inmates at other prisons than male inmates raping each other. That's rarely brought up when talking about prison rape however. Not as controversial I guess...