Justice Kennedy’s opinion for the Court argues that there is a national consensus against executing child rapists, as evidenced by the fact that only six states have provisions for such executions. Justice Alito’s dissent points out that one reason few states have such provisions is that in an earlier case, Coker, the Supreme Court had prohibited the imposition of the death penalty on the rapists of adult women–and called into doubt whether it would permit executions for anything other than murder. One act of activism thus provides a spurious legitimacy for another.
(I would rather that state legislatures repealed their death-penalty statutes, and I think a reasonable argument can be made that if such statutes stay on the books they should be limited to homicide cases. But those are arguments that ought to be made within the legislatures.)