Tl;Dr at bottom
As a heads up I have not read the Manga so take my answer as what you will.
Through out the majority of the season we've seen Legoshi come to terms about how he feels about Haru and how his mind and body conflict at times. Legoshi went from Haru is food, Haru is not food, Love Haru, Haru is prey, essentially figuring whether he felt the way he did because he either liked her or wanted to eat her. He however reached his conclusion that he liked her.
Haru I would say has not yet gone through all this development and is either only just starting to or is in the midst of it. She has only really accepted that somebody likes her for herself without pitying her and just thought that that is enough, when reality her body is screaming that something terriby wrong.
Conclusion, while I don't necessarily believe that the action of putting her arm in his mouth is intrinsically a defensive mechanism, its more of a manifestation of the conflict that she has yet to/currently going though.
Body: want to run,
Mind: I like him though,
Body: PANIC DO SOMETHING WERE GONNA BE Eaten,
Mind: hells ye we are ;),
Body: SMURGAFLEEEUR ARM TAKE, THEN LEEEAVE