Shounen, seinen, and the like are all demographics based on who the particular show is aimed at. However, it doesn't indicate that boys won't watch a "girly" show and vice versa. Black Butler runs in a shounen magazine, and it definitely has a strong female fanbase. Labeling something as "shounen" just means it's aimed at boys. In my opinion, it works better with manga magazines, just because it more or less says "-insert genre/plot/etc.- will not be in -this magazine- in any large quantity. An angst-filled romance will not be in Shonen Jump. The only series that seem to be able to pop up everywhere are ones like Chi's Sweet Home, which carries universal appeal and runs in a seinen magazine.
That said, I find it much more difficult to find a particular demographic for most American animation in Japanese terms. Shows on CN or Nickelodeon have an appeal to both males and females, even if lacking female characters. The Simpsons, Futurama, South Park, and others are comedies targeted at anyone who enjoys them. It's not about a serious adventure and none have several raunchy scenes depicted seriously and/or meant to appeal particularly to men or women. A lot of it is day-to-day activities, politics, or satire/parodies of life.
With the exception of children shows, adult animation is very broad in appeal, partly because it has to be, I'd imagine. Not as many adults will be watching cartoons compared to children. You'll always have a bunch of girls willing to watch Strawberry Shortcake or Holly Hobbie, and the rest of the "male" shows end up attracting a fair share of females (it's also not explicit that it's aimed at males in the first place, while most anime is fairly easy to label). There a few exceptions, I suppose--Avatar could likely be called shounen.
