Any perp covered by this law would have a *very* strong incentive to kill his second victim.
Also,I'll wager that the SCOTUS would strike it down as "unusual",meaning that the punishment is not in proportion to the nature of the crime,as is the case with executing a murderer.
So we add a law for that contingency: death by boiling in oil.
Actually, there's a second reason as well: It will reduce reporting of sex crimes against children. Much of this stuff that goes on is from a family member or family friend. Many people might be willing to send creepy old Grandpa to the slammer for a few, but are they willing to see him die for it? Many people would think twice before involving the authorities, if the perp was someone close to them.