My reasoning for Clark's behavior was not so much that he was less of a lunkhead, but that he doesn't want to hurt Lois's feelings any more than they're already hurt. It also struck me that Lois's defense of Luthor sprang partly from the (somewhat adolescent) wish not to be put in the wrong, especially by Clark. If he'd thought about it at all, he would have realized that her competitive nature wouldn't let her back down, so the key to getting her to actually look at Luthor was not to put her on the defensive.
Nan
