Personally, I've always had a fondness for castration as the penalty for rape.
Capital punishment would be saved for repeat offenders (Gotta admit, you'd have to be pretty bad to rape without the, erm... equipment).
Virtually 100% of rapes are committed by men, so the rare cases of the opposite hardly need accommodating for.
Want to keep your manly parts? Then you'd better leave the ladies alone when they say "no".