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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Definitely not a mental disorder or a choice but just the sexuality they're born with. There is nothing wrong with liking the same gender as yourself.