The retitling was specifically done because girls won't go to see John Carter of Mars and boys won't go to see A Princess of Mars, in both cases because they don't know what the movie is really about. A sequel won't suffer that perceived branding problem, so would probably be titled something like John Carter: Warlord of Mars.
The poster has an almost bare-chested guy looking broody. There are plenty of pictures of an exotic-looking beautiful warrior princess kicking ass.
Trust me, I work in marketing and I'm pretty sure both markets are catered for, image-wise - thus making the removal of the words that clarify the genre all the more baffling. It presumes that people still go to the movies and take pot-luck in the foyer without knowing anything about the movies available - which no-one I know can afford to do.
And if you think girls won't go and see sci-fi, you need to check your local movie-theatre more closely!