Fewer things in life are better than butts. Glorious, round, in-your-face (not literally) butts. And when it comes to movie posters, the advertising people seem to have grasped this concept whole-heartedly and decided that the best way to put butts in a theater is to put butts on a poster. Which is clearly why the theme shows up again and again on movie posters.
Need proof that there are all kinds of butts on movie posters? This list is the place to see. The main butt construct for posters is the subject/main character looking over their shoulder into the camera like they're having a sneaky and/or private moment with the potential viewer. Don't be fooled! You're there to stare at some cheeks, kid. There's also the between the leg looking in/out shot, which makes you wonder what the hell this person is doing just standing around with their legs spread for. Are they waiting for a child to fall out? WTF?
Regardless of the motivation of the characters on these sexy movie posters, the fact remains that butts on movie posters is a thing, and unless you're a boob man, it's a glorious thing. And for the man butt fans, there are even some film posters with guy butts in there too.
http://www.ranker.com/list/the-greatest-butts-movie-poster-history/john-barryman,
Æon Flux
John Tucker Must Die
Lara Croft: Tomb Raider
Undercover Brother
G.I. Joe: The Rise of Cobra
The Avengers
The Dark Knight Rises
One for the Money
All Cheerleaders Die
Sorority Party Massacre