that's a lot of words. allow me to summarize your argument, and please correct me if i've misunderstood.

A. humans are rational creatures
B. rational creatures make rational choices, unless influenced by madness or sexual preference
C. being otherkin is a choice D. choosing to be otherkin is an irrational choice E. otherkin are not mad ∴ otherkin choose to be otherkin due to a sexual preference

your conclusion does seem to follow logically from your premises, though I don't think i agree with the truth of most of those premises.