Well, ABC was worried it would kill the show to have them married ... take away all of that UST and what do you have?
That's easy - you get seasons three and four. :p
In my ideal LnC world, Clark wouldn't have proposed at the end of season two, but simply confessed the truth to Lois. Season three should have been them trying to figure out how to carry on a romance that has three people in it (her, him and Superman) while they both adjust to the fact that she's in on the secret. And then they could have been engaged/married in season four.
ETA: Most of all, though, the show died for me because the characters were no longer the ones I'd fallen in love with. They both became cartoons of themselves starting with season three. I very much prefer the sharp wit and banter of the early days over the milquetoasts they turned into.
