why do sex offenders get less good time? They will be punished for the rest of the lives as it is once they are out of jail.

They do NOT get less good time. All inmates get the exact same privilege - it is up to them whether they keep it. Inmates are granted a 15% good time credit the day they are incarcerated. Only the inmate's failure to follow the rules would cause them to lose any or all of this privilege.