This would be an exercise in futility.
A psychiatrist would tell you that to cut off the sexual drive of an individual, you'd need to perform the castration before the man enters puberty. After that, the sexual drive cannot be said to be the sole result of a couple of nuts. There is an intricate little chemical play in the man's body, and the brain responds to this.
Since sexual offenders usually aren't just interested in getting off (in which case masturbation or potatos would suffice), they'd be likely to continue whatever they did before, but in different manners. If they're turned impotent, their desires and frustrations would still be taken out on victims.
If there was some kind of purpose behind this punishment, I would be behind it. But the science behind the effects seem to show that it has little effect on the future behavior of the offenders - *unless* the offenders themselves request the castration, as has been the case in Denmark and Sweden.
'Chop it off' is my immediate reaction. in fact, the 0older I get, the more violent and aggressive my immediate reactions are getting. I'm becoming more and more intolerant. Must be some diddlyed up toejame in my childhood

. Anyway, despite the first reaction, which IMHO is fully understandable, I think it's a good idea to determine the effects of an act before carrying it out.