This is a tough one...
I don't think cutting off sexual offenders penises will keep them from harming anyone ever again. They will likely try to repeat the offense, seeking some form of pleasure, even if they can't obtain it sexually. Rapists are violent, not just sexually.
If you were look at it as an 'eye for an eye' situation, you could castrate and throw them in jail, being the worst punishment.
I guess I would allow such a law, simply because I think sexual offenders should have the worst punishment possible.
"Noodles, the man who put the O in Obrigado." - Dexter Holland
"You're a sexy fucking audience!" - Noodles
"It is the power of music, to carry one directly into the mental state of the composer. The listener has no choice, it is like hypnotism." -Beethoven