Revealing mistake: There are numerous times throughout the series where the supposed big city of Metropolis is actually just a back lot set and none of the buildings are more than a couple of storeys high. Quite often the green hills of Southern California can be seen beyond the set and between the buildings.
Revealing mistake: When Lois and Clark are checking out Lenny Stoke and Derek Camden on Lois' computer, you can see the computer screen showing the script for Lois in that scene. (00:07:40 - 00:08:20)
Answer: I don't recall if it was ever mentioned in an episode, but it's possible that the frames and/or lenses of his glasses are lined with lead, the one compound his X-ray vision can't penetrate. Speculative, of course, but it likely would have been done at a point in his youth when he wasn't in complete control of his powers.
Cubs Fan ★
I think the other answer has good speculation. I would add it could also just be an "acting gesture" that Dean Cain utilizes as part of his Superman character and not because there's any practical reason. That way, the audience immediately recognizes what he is doing.
raywest ★