You have to excuse me but i just have to respond to the To Love-Ru (Darkness) comment with this: Tokyo's Youth Healthy Development Council Looks at To Love-Ru Darkness
The japanese goverment decided not to ban it but it came close. SJ friendly indeed and the Bill is still in effect. Knowing is half the Battle!
Regarding japan´s "open" attitude to sexality i would like to point your attention to news stories like this. Why have young people in Japan stopped having sex?
A more sexualy repressed first world country barely exists...

My two friends posted the good old sophistication points of adult vs youth media but Barefoot Gen is still Shounen and there are these gem which have no demographics but are for the whole family: "The Diary of Anne Frank" (1995) from Madhosue or "Grave of the Fireflies" (1988). Modern classics as FMA are heaps grown up too.
What interests me more is why RPG adaptations as Hitoshi´s Utsurowazarumono - Breath of Fire IV (girls love Tales games so that makes some sense) or Durarara!! - 3 Way Standoff - Alley came out as Shoujo. BoF especially makes zero sense as it was 10 years old so selling it to a new audience was futile. Oh well. I gave up fo figure this mess out years ago as there is no logic.
Edit: @Thailin Thank you and well spoken. That is how you tag.

I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb

As others have said, it has entirely to do with the demographic for the magazine that it was published in... otherwise I could make things even more convoluted by pointing out that there are a few series in Comic Zero Sum that always seemed kinda shounen to me, such as 07-Ghost, but it's really all josei.