Hmmm, how about if Lois and/or Clark decided to investigate a group that's "for married people only" or something? And they check out newcomers thoroughly, so no fake marriage certificates.
Or maybe...in the US, a wife cannot testify against her husband (and I assume vice versa), so maybe one marries the other in order to keep their mouth shut...or else to keep themselves from being obligated to tattle.
Personally, I've always wanted to put a Kryptonian spin on the whole "I didn't know that made us married!" thing.
