A preteen falling in love with a senior officer could have been too awkward
No, as I understand it, the BBC rejected "Miri" for the insanity and disfigurement of the disease, because they had pegged Star Trek as a children's show, and the episode violated their rules for such.
You know what, I agree. Miri was an episode that had a lot going for it, but the creators made a lot of weird choices.