User blog comment:Brandonmatias/Yami's "dis-likement" to Ecchi things./@comment-7948498-20140920073501

It was never explained. But the source of her fear of tentacles comes from her past experience of being on a planet full of tentacled creatures. In anime and mange, tentacles are really weird toward girls. Perhaps her dislike of ecchiness is somehow related to that. But otherwise, it still hasn't been explained so far.