Classification boards don't like 'anime sex' because, let's face it, a lot of anime sex involves characters under 18. These characters are actually adults so that shouldn't be a problem.
You're absolutely right. However (at least in Aus), the character only has to appear
under the age of 18 for it to be illegal. And exactly what that means is open to interpretation. So even assuming Catherine is in her twenties or thirties, if out classification board decide she looks too young (which they often do with anime) it'll be kicked.
Not a good example for anime, but look at how long this list is:http://en.wikipedia.org/wiki/List_of_banned_video_games#Australia
and, of course, that's only the games that companies have actually tried
to have released down here. There have even been calls to ban Mass Effect 1 and Modern Warfare 2, but both were released without any censorship in the end.