The poster is rather lewd.
Two guys holding each others asses? It's not pornographic by any means, but it doesn't leave anything to the imagination either. The prospective viewers already know exactly what the movie is about without even seeing a movie trailer, that's how descriptive the movie poster for Bros is.
Advertising still has a big impact. Marketing for this movie let people make up their minds about it just by looking at the poster. That's very bad. You want people to be intrigued, not just give the game away.