I don't like to be the bringer of bad news but...
I too agree that those two 14 foot room dimensions are going to prove to be a serious detriment to getting good, smooth bass response in that room and I'm not so sure that closet will make a huge difference as the low freq waves might easily pass through those glass doors... however another problem with having huge glass anythings in the room is what it will do to sound reflections at the higher frequencies.... additionally its asymmetrical to your direction of intended layout. Bottomline is your particular listening space I'm afraid appears its going to offer some serious sonic challenges to your efforts to obtain good imaging and a smooth freq response. You will have to rely on judicious speaker placements and lots of room-treatments to try to get the best from your proposed listening environment as it currently exists. This is just my opinion. If it were me, I would actually be considering ripping out the closet's glass doors and replacing it with an actual wall or hanging some very heavy doors in place of the glass at least.
The short answer is because the length of travel for reflections will be multiples of the direct radiating wave which can often lead to complete room nulls (and boomy peaks) ...its because so many of the dimensions of travel across the room will be multiples of the straight line distance to your ear. This is also why a listening position in the center of the room is also inadvisable. Once you begin your search for a sub position that doesn't result in complete nulls you will come to the realization that one really doesn't exist for listening positions on a boundary or in the center of the room. That's why the suggestion is given to select listening positions that are at 1/3rds of a room's measured dimensions (really its 38% and 62% that are considered best). In your room that would be ~5'4" (38%) and ~8'8" (62%). The first dimension I would think would be a bit too close to be considered a reasonable viewing distance (unless your screen is really small) and so 8'8" would be the best place to locate the futon with the hopes of finding a reasonable place for the sub that's free of complete nulls (and boomy peaks too). Also if you plan to do any sort of rear surround speaker placement, then a listening position up against the back wall would destroy any directional sonic value those speakers might have provided. Bottomline... placing the futon against the back wall is not really a good idea.