They could have flipped the genders, also. Dumb marine gal goes native, falls in love with hot athletic Na'vi guy who is supposed to be hitched to this stiff, boring girl.
You can't win. If they had reversed the gender roles in this movie, everyone would have been in an uproar saying "Oh, of course, the know-it-all male is showing the ignorant female that her way of life isn't good enough" or something like that. Seriously, sexism can be seen in anything if you look hard enough.
In Hollywood films, one rarely has to look hard.