For those that are for retributive (American-style) rather than rehabilitative (Scandinavian-style) justice, this should make perfect sense. A rapist did not kill anyone and by SCOTUS Kennedy v Louisiana and Coker v Georgia, they can no longer be put to death anyway (which made no sense if they killed no one). If they did, then they would be charged with both murder and rape. If a rapist seeks retributive justice, then the most that should happen is that the convicted be raped, perhaps in view of the victim (as they do in death penalty).
I personally don't support the death penalty for any crime, so I don't agree with the subtitle of this opinion piece, but that's besides the point. I get it, rape is an absolutely horrible crime and rapists should be punished for it. However, it's simply hypocritical to say that rapists should receive retribution by being raped. It shows that rape is an adequate punishment, when really, we have no proof to support this. Rape is a serious issue, so why should we use it to emotionally and physically harm others like the rapist did to their victim?
There's a basic flaw here. When someone is put to death, they will never kill anyone ever again. But a rapist could decide that being raped themselves is worth it to rape someone else. You could even end up in a situation where you had a rapist that enjoyed both forcing others and being forced themselves (rapists are by definition pretty sick people) and therefore the punishment would become an incentive rather than a deterrent.
The rapist could be cruel and enjoys raping the prisoner. Although prisoners are often raped in prison, that probably is not punishment. There it would be unusual punishment.
If a murder dies in a gas chamber, living in the modern world we must all breathe polluted air, so that would not be cruel or unusual.