he's not making a rape joke, he's referring to the fact that prisoners tend to treat convicted rapists poorly, on top of the fact that he's a cop, that means he will not be treated well and probably will get mercilessly beaten by his fellow inmates on a daily basis.
I'm aware of the brutalities of American prisons. It's part of what this page is opposed to and aghast by. Rooting for it and chuckling about it is part of the problem.