Reproduction by cloning is the most sophisticated manipulation of nature in history. It lies on the other side of some imaginary line dividing acceptable medical technology from the unacceptable. A cloned human would not be a Xerox copy of another human. Identical twins have the same genetic similarity as a human and his clone, and yet we do not say that only one of a pair of twins is human. By this, they conjure up images of Dr. Frankenstein, who created life where there was none.
Cloning creates life from life.
They define the essential element as the combination of DNA from two people. A hundred years ago, when DNA was unknown, people thought the essential element of natural reproduction to be the act of intercourse. In vitro conception would have been considered as unnatural and sacrilegious as cloning. Already, some have imagined the possibility of cloning oneself in order to rob the clone of needed organs. You cannot say that quantum physics is an immoral discipline simply because it allows for the creation of atomic weapons.