Some people are into reading 'for the porn'. If they want great literature, they'll visit the library. Other people want a little story with their porn, but the focus is clearly on the sex. Some want a well written story and the sex is merely the icing on the cake. I'm in that category. I've read plenty of stories with little or no sex, but my preference is a nice balance, with the character's sexuality being included as a normal part of the story. In the case of the HP books, there was no focus on sex, likely due to the age of the expected readership. However, sexuality is huge in the lives of teens. They are highly hormonal at that age and ignoring it makes no sense. Especially older teens; if they aren't having sex they are dreaming about it, thinking about it, talking about it, anticipating it, dreading it, reading about it and watching it. Older people shift their focus to career and then family, but hopefully sex remains an active part of their lives. Readers are fascinated with sex in all it's shades.

For me, the main thing is retaining the quality of the story in the sex scene. Some people can't write good sex scenes and it might be better to allude to it rather than try to write it and have it come out poorly done. Sex is often written in a mechanical way and the reader ends up tuning out. I like the human factor in the sex scenes, the emotional content. That is what I love to read. A sex scene that engages me emotionally and makes me feel like I'm there.

So that's how I feel about sex in fiction. If you're going to do it, make it good, but don't go overboard or write it in a way that has the reader skimming it to move past it.