I don't think anyone really knows about the true atrocities that actually go on in prison, unless you've been or have 24hr surveillance.
There is no chance of pregnancy in male on male rape either, or when a man rapes an 82 year old woman, or someone who is infertile, so I'm not sure why that is relevant in determining the definition of rape.
US class it as rape and I'm inclined to agree. I wonder how other countries define the term.