Good Question: Paul speaks later in the chapter about a dual nature- we have a sin nature which we inherited from Adam, and a new nature. You could think of the sin nature as a badly mutated nature that wrecks havoc with our human nature, which is created in God's image. Our old nature that desires to sin is what is put to death. Also, in relation to the 'marriage' comparison, if our old nature is dead, it is no longer 'wed' to the Law.