You over simplify the situation. One law exists to protect children from being coerced. A maturity line needs to be drawn or the law becomes completely ineffective. Its a matter of necessity. The death penalty being imposed on the rape of a 12 year old, but not the rape of a 13 year old, does NOTHING.
Also what if the person thought she was 18. You are going to kill someone based on strict liability? Come on...
That study is horseshit.
I think the best way to characterize your position is to say there is credible evidence to suggest the death penalty doesn't work. To declare that it doesn't as an absolute fact is a bit misleading.
Having said that, we've both made our cases and I think it's best to agree to disagree. As I said before, this comes down to a basic difference in worldviews.