I am also of the "they should be together" school, but, even so, here are some additional thoughts in the other direction.
Possible scenarios for Lois to lose Clark and move on:
1 - Clark dies. However, he is difficult to kill and death by natural causes isn't likely until he's an old man.
2 - Lois decides that she doesn't love Clark, marries someone else *and* (because "moving on" implies contentment) lives happily ever after. Not likely because we tend to believe that she would eventually come to regret letting Clark get away, which doesn't equal contentment.
3 - Clark leaves Lois. Not something we can really conceive of, with the exception of the NK arc. The entire basis of this fandom is that Clark loves Lois. We can take out the Planet, mess with the superpowers, pretend the Kents didn't exist, or prevent Krypton from blowing up, but a L&C fanfic in which Clark meets Lois (the one from his own universe) and doesn't eventually love her (or in the case of a bad Clark, have a cruel fascination with her) is very, very rare.* So the idea of him leaving her is a very hard concept for a FoLC to swallow.
(*Although one example was pointed out to me: "The Road Taken" - very good story - but even in that one there was an attraction and the future was left open ended. I have hopes that Terry will write a sequel to this one eventually, even if it involves L&C *not* getting together.)
Possible scenarios for Clark to lose Lois and move on:
1 - Lois' death. Much more likely than Clark's death because she is human and has a knack for trouble to boot.
2 - Lois' leaves/rejects Clark and possibly marries someone else, either happily or unhappily. Obviously there is precedent for this.
The two "Clark moves on" scenarios are more plausible in my opinion than the three "Lois moves on" scenarios. So, assuming there are indeed more "Clark moves on" stories, maybe that's why.
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QofC, I like your thoughts. I agree with you (and Tank) that Clark would have a harder time moving on, so I guess there is more to explore there.
