Personally, as far as rapists are concerned, I'd set the penalty as a penectomy, along with castration. This is not as cruel as you might think, because you are simply "disarming" the rapist of his weapon. Just as you would not release a robber or murderer and re-arm them as they leave prison, why would you leave a rapist "armed"? Watch the rape statistics dwindle to almost zero, especially since recidivism would be zero!