well i've got two thoughts, the first being under the 'render stats' portion of your object (not your material) in the attribute editor there's a checkbox called 'double sided' that is checked. not sure what it does, but give that a try.

not being a pro with UV sets yet, (oh hell i'm barely experienced with them hehe) instead of giving you a UV workaround i'd say why not just take a box and scale it really small in only one axis so it looks like a plane? that way you can texture one side of the box, and the other side, it'll look close to a plane, and you'll be able to have a texture only on one side.