Castration
Because rape is a serious felony, the penalty for those who will commit that act should be death. Giving death penalty to a guilty person means that justice is served. It shows also that the justice system values the dignity of rape victims. Further, death penalty for rape would deter crimes related to such as people would think twice before committing that crime, otherwise they will be penalized by death.
I am a strong supporter of the death penalty, but in this case I believe that it may be too lenient. Death by hanging or electric chair is relatively easy and everything will be over in less than one minute. However, their victims don't have that choice. They will suffer from discrimination and mental trauma for the rest of their lives.