The point is merely that if the undesirability of a living fetus is a just ethical ground for its destruction, the same principle would apply to other living humans. In other words, if it is unjust to kill a three-year-old child or a three-day-old child because he or she is undesired, then it is likewise unjust to kill a living human before birth. The bottom line is that undesirability is not a just moral basis to kill a human being.
