Bit of a mystery here, with several possible causes, but firstly I would make sure it is not a trick of a twisted rear mudguard.
The front and rear tyress have different profiles, so the straight edge should not touch either side of the front tyre as Jules correctly states.
Each wheel should be located centrally, on the imaginary datum line passing along the central spine and steering head, in other words, centrally between the forks and rear chainstays. Moving the rims sideways is a case of adjusting the offset of the rim to the hub centre by spoke adjustment.
The hub position sideways is governed by the fitment of the correct bearings, hub spacers and internal sleeves.
If the front wheel is considered to be OK, then the rear wheel must be slightly off the vertical when viewed from the rear. As a first check I would try removing the rear wheel, and reassemble the spindle and see that it lies horizontally. If it is not level the fault is in the plunger assemblies, or the position of the plunger mountings themselves, due to a misalignment in the frame.
It may be that the plungers have mis matched springs or one side is binding. Whatever you find, the easiest fix will probably be making custom spacers to return things to normal.
Somewhere on the forum are details of frame dimensions, which will help you determine if you have a major mis alignment. The upper chainstays should be symetrical on each side of the seat lug at the top of the vertical frame tube. The through stud here passes within a sleeve which is not listed as a spare part. The sleeve ensures correct alignment between the front and rear sections of the frame.
Hope this is some help.
Swarfy.
Rex, I have always thought the lower tubes were symetrical, well on my frames they always looked that way.
Swarfy