Anachronism said:
If having a body double bothers him, couldn't he have just done the nude scenes himself? It's what 90% of the female cast have to do, as far as I can tell. Frankly, GoT needs more male nudity, and considering the fans' fondness for Mr. Harington, I don't think any of them would have complained.
Seriously, Game of Thrones needs more cock. Not even kidding.
I believe the double is primarily for action scenes, the actor's in good shape but that's not necessarily actually the same thing as being athletic or skilled enough for some of those fight scenes. Also, I believe that at one point he was injured.
This isn't unusual, stunt-work and stage combat are an extremely technical and focused specialty to the point that few of the specialists are actors to begin with, and those that are are usually obsessive enough about the technical parts to prefer not to waste their time being the guy doing the lines and brooding at the camera as well.