You pose as a bike messenger by acquiring a fixed-gear bike that you can't properly ride. You can spot these kids immediately because they are usually pushing the bike - and when not pushing they are at the point of colliding with someone. ;-)
I get that part. But ultimately being a bicycle messenger is a job... buying a piece of equipment associated with a job doesn't make you a poser necessarily. Plus, fixed gear bikes are not exclusive to bike messengers.
As for learning to ride the bike, you have to start somewhere... this would make everyone learning how to ride a bike a poser.