Justice is not vengeance. Mutilation is not a valid form of punishment. If we follow your "logic" then thieves should have their hands cut off. Escapees should be hobbled. Peeping Tom's should have their eyes removed. Politicians caught in lies should have their tongues removed... you know to make them think twice. Sexual Counseling is a thing that exists in the real world. It is possible to actually treat some sex offenders in such a way that they can rejoin society that doesn't include you cutting their balls off.
What screwed up world do you live in, as a small government republican, where you think the government has the right or capability to make good decisions about permanent surgical punishment? There was a story a few weeks ago about corrupt collusion between police, attorneys and judges sentencing innocent people. What protections do you propose for this law? How will you protect the innocent from your burning need for blood sacrifice to atone for crimes? Which sexual offenses will this cover? Rape? Does that include statutory rape? Is it just anyone on the sex offenders list? I know you say its just for people over 21 involved with children under 12, But how long before it expands? How long before your bad thinking leads to another crime fitting your punishment, then a new punishment to fit another crime you find too vile for normal justice to be sufficient?
We do not use our justice system to dole out revenge. We punish through taking away freedom, not altering people physically.