This would be my approach.

1) First Offence, Jail, Monitoring for a period of time
2) Second Offence, Castration is voluntary option, Jail (3 x longer), Monitoring for 3 times the length of time
3) Third Offence, Castration is mandatory, Jail (10 x longer), Monitoring for the rest of their life

A bit like 3 strikes and you are out. Harsh but necessary to protect society.