Rapes inside of prison are almost all male on male, and there is evidence suggesting that the numbers of such are comparable to and may exceed rapes outside of prison. So it is not impossible that the majority of rape victims in the USA are men.
It's not like the issue of men being raped is predominantly due to women, whereas the issue of women being raped is predominantly due to men.
I also wonder if all the 'alpha male' bullshit might actually be a detrimental influence on prison rape, since it so clearly reinforces and mimics the sort of hierarchy that exists among men in prisons.