Rape doesn't necessarily involve the rapist using his genitalia. It's about him having the power and could just as easily be performed by someone who's been castrated. Therefore castration = revenge. I'm not saying revenge is a bad thing. It might provide a much needed panacea for the victim and the rest of society, I'm just saying it's revenge and rather call it what it is.

Edit
It's always interesting to see the reaction of people to a rapist compared to say a drunk driver who kills one or several people. In both cases the victims are innocent bystanders. Why is it that there's no calls for drunk drivers to be cut open as a public spectacle or maybe have their stomachs/arms removed so they can't recommit.