Male Protagonists in Shoujo Manga

Though most shoujo manga typically have a female protagonist, there are some that have male protagonists. With a female protagonist, shoujo manga also usually have an underlying romantic development part of the plot, which isn’t characteristic to a shoujo manga with a male protagonist. What is it about these male protagonists that classify the manga they appear in as “shoujo”? What are the similarities and/or differences of various male protagonists of shoujo manga? How do their interactions with other people, their surroundings, and the plots of the manga determine this classification? Finally, does having a male protagonist in a shoujo manga have any social implications, since the target audience is generally teenaged girls? If so, how?

A few examples include: Hakkenden, Natsume Yuujinchou, and Gakuen Babysitters. I’m sure there are others, but these are the ones I remember off the top of my head.