So what is the unmet need behind an obsession with sexless/child-like figures?

My guess is unfulfilled (genetic) longing for a "real childhood", where a child does not have to take on adult roles. This means available, stable, supporting and functioning parent figures.

Not having fully experienced that leads to "holes in roles", which want to be addressed and re-nurtured one way or the other.

Could it be as simple as wanting to maximize the number of customers you reach by writing the movie around a PG-13 rating?

there are countless ways to depict sexiness that wouldn't bump a movie from PG-13 to R. even something as simple as a slight eyebrow-raise in the right hands can be sexy