God did not say that if he touched it, he would die. In the following verses we will see what thought does, and where it has led man. Abraham's thinking produced fear, and his faith was weakened. This led him to lie about who his wife was. Yes, she was his half sister, but a small or part lie is still a lie.
Most lies come out of fear.
Fear is God's faith twisted to become Satan's faith. Even out of evil thought comes good, if God is in charge. Turn from thy fierce wrath, and repent of this evil against thy people. And the LORD repented of the evil which he thought to do unto his people. I thought on my ways, and turned my feet unto thy testimonies.