While fantasies are intrinsically harmless, and can help to manage fixations which might otherwise disrupt normal social functioning, the increasingly prurient manner in which you apparently choose to exorcise your preoccupation with this girl suggests that your 'friendship' may not be as genuine as she thinks it is, and moreover that your own perception of her may become distorted as a result of this pervasive (and as yet, unrequited) lust.
So yes: 'friends-with-benefits' is probably a subject best broached sooner rather than later.