Either:
A. They are born or Genetically rewritten to be attracted to the same sex, thus being counterproductive to nature and all natural processes.
Or
B. They choose to be that way and promote the 'Sex Magic' that it is, thus purposely opposing natural processes and Nature, similar to Cannibalism .
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Neither. In the DSM V they arent listed as mental disorders and its definitely not a choice. Who would choose to be gay in countries where its punishable by death?