In short, Avatar has heterosexual themes, that's why. Not that hetrosexual themes are anything new in American films but when you consider the context in which the Avatar story is set, the message of this film is highly irresponsible.
Here's a summary of what is wrong with the Avatar film:
1) Avatar assumes that the heterosexual, "male" and "female" attraction will still be the primary relationship basis in the future.
2) Avatar ignores the fact of Evolution. Humans are evolving to be being Transgender, NOT heterosexual,
3) Avatar also assumes that intelligent beings on other worlds would be hetrosexual.
In addition, the film uses militaristic themes and promotes violence as a means of story-telling and social context.