That is not true these rapists cannot help themselves if they wanted to, the punishment is the last thing on their mind when they commit this crime.
Dude, does it really matter to a rapist if it's 10 years in prison or his head chopped off. They are both strong deterrents.
However not all rapists are sexually repressed creatures who "can't help themselves", some people also make mistakes. Should the person who made a mistake once of raping someone and is not likely to do it again have his life taken away?