Eugenics is unethical
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Eugenics is an effort to play God
We should leave the human race's genetic design up to nature. When people use genetic engineering to alter humans, they are deciding perfection, which should not be left up to anyone.
< (2 of 3) Next argument >
The human race's genetic components should be up to nature. They should not be based on the parent's preferences. Since the definition of perfection is subjective to each person, it is impossible for a person to truly decide what is best. If parents are able to choose genetic traits for their child, this will inevitably lead to multiple standards of perfection. Due to the subjectivity of perfection, people will likely be influenced by their own environment, culture, and feelings. This can lead to a culture’s preference for one trait and prejudice towards another. By playing God and creating what parents prefer, they are fostering an environment that looks down upon other traits. In extreme situations, this could mirror the abuse of eugenics in Nazi Germany. 
A person's preference of one trait over another does not necessarily mean that they have a prejudice against those that do not match them. While Nazi Germany was focused on eliminating groups of people as a form of discrimination and negative eugenics, this is not the case for someone that prefers a physical enhancement. Humans should be given the ability to genetically alter themselves through eugenic procedures such as gene therapy. If all of human life was left up to nature, this would eliminate the use of any medical practices that prolong life. Eugenics has the possibility of eradicating certain diseases and prolonging a person's life.
[P1] Perfection is subjective. [P2] Deciding perfection of the human race is a form of playing God. [P3] Playing God is unethical. [P4] Therefore, eugenics is unethical.