Well, after Superman II and the "memory kiss," Lois is
supposed to have forgotten that she slept with Superman, AND that he's Clark Kent...that's the bottom line, the status quo restored, end of story.
But for Superman Returns, Singer just picked what he wanted to use out of Superman II to make his story work, ignoring those previous, MAJOR plot points. For his story to work, she needed to remember one out of those two little items...so he made that happen, thereby making the cluster of the Superman franchise even worse. (So in Superman Returns, Lois should just be really, REALLY confused as to why her son has superpowers...she shouldn't have remembered sleeping with Superman...but Singer doesn't care about story apparantly...)
What did he say when asked about the memory kiss from Superman II? "Oh, I just ignored that part...I assumed she remembered sleeping with him." Singer makes me sick. His "vague history" was so vague, even he didn't even know what the hell was going on.