io9 has an inteview with one of Avatar's concept designers.

What about the sex appeal aspect of it? Was the sexiness something James Cameron emphasized with you?

Well, he wanted them to be very beautiful. And I do believe that, at some point, he said something to the effect of... the audience has to want to fuck her. I mean, Jim is very plain in his language.

So, I went, "All right?" So I made something that, I don't know if I really particularly wanted to fuck it, but it was certainly a beautiful alien. He definitely, he wanted it—because he really prefers women that are kind of athletic, and buff and stuff like that, so I, you know, designed something with big hands and feet, a big presence that felt really big and strong.

So it was designed for his personal preferences in terms of sexiness?

It certainly wasn't mine. I mean, I would have sculpted, I don't know, Gretchen Mol or something.