I've always been someone who can see Clark justifying not telling Lois the secret because he believes that Clark Kent is dead, and thus there is no point in telling Lois he's not, because he truly believes that the person, who was her partner is no more.
Better to let Lois go through the normal grieving process, and get on with her life. To me, that's a rational, and justifiable rationale.
But, we all know that a revelation has to happen so together they can figure out a way to ressurect Clark Kent. This, of course, leads to the expected troubles between the two, which after heartfelt conversations, explanations, and groveling, finally leads to the happy ever after.
BUT... in this case, that can never happen. No matter what rationale Clark may argue for witholding the secret, Lois will (nor should she) ever forgive him for his duplicitous behavior in sleeping with her. It doesn't matter that Lois was just as much the aggressor. She wasn't privy to certain vital information which would alter the scenario in her mind.
Lois' actions were tacky and less than honorable, but Clark's were unforgivable.
Tank (who thinks it would be a nice change to have a fic that doesn't have a happy ever after ending)
