The root of the 'problem' (which I don't think is a problem at all) is that using 'shounen' or 'shoujo' for genre titles has nothing at all to do with what's in the series. The reason they get called that is because of the magazines they were originally run in. Those magazines cater to certain demographics, of which 'shounen' (young males; usually the 10-15 crowd), 'shoujo' (young females; again, likely the 10-15 crowd), 'seinen' (older teen/adult males), and 'josei' (older teen/adult females) are the magazine's target audience. Sometimes the demographic name is in the magazine name, but often times it's not. Not that I think I needed to say it, but magazines are how manga and light novels are largely first run in Japan, with single-series tankoban compilations coming later. The racks in the store separating the selections by age and gender is most likely where the rigid idea of this all originates. Heck, you'll even see this happen in regard to American bookstores. When was the last time you saw Ladies' Home Journal or Shape or Cosmo interspersed among Men's Health, GQ, or Maxim that wasn't on a grocery store endcap? And I really mean
interspersed, where there is no distinct 'here are the women's in one grouped section, and the men's are in another group right next to them or maybe a section down'.
Segregating the series into magazines for a specific age/gender grouping tends to cause a positive feedback loop where certain common elements and themes may develop amongst the magazines' culture (a case of authors knowing their audiences, too), but even at that the overarching idea of 'romance series', 'action series', 'comedy series', 'drama series', and so on are not relevant at all to what demographic it was meant for. What distinguishes them is usually
how they do it, not
that they do it. My point about what happens when it jumps the Pacific is that American culture generally highly stratifies along plot elements (i.e. boys don't want romance stories, only action; girls don't want high action, only romance*), and we don't have a large magazine comic syndication market that would recreate the Japanese divisions in content (and even if we did, it might still fall prey to the aforementioned plot segregation).
*Despite all the evidence to the contrary, really. My sister goes after action stuff 95% of the time.
Basically, I draw my definitions for 'shounen', 'shoujo', 'seinen', and 'josei' from the original sources and from the accepted categories they get filed under on ANN and Wikipedia (which again, usually use the original sources to determine their categories).
http://en.wikipedia.org/wiki/Sh%C5%8Dnen_manga
http://en.wikipedia.org/wiki/Sh%C5%8Djo_manga, especially the
Western adoption section
http://en.wikipedia.org/wiki/Seinen_manga
http://en.wikipedia.org/wiki/Josei_manga