I think PML was fairly conclusive in saying that Lois was attracted to Clark while not at all to Lex. So she never really "fell" for Lex. She was following her head and not her heart.
She basically settled because she wanted stability. Lex understood that, which is why he went to the trouble of destroying the Daily Planet. He knew Lois would never have accepted his proposal otherwise.
Despite being supremely confident outwardly, she was a vulnerable lady, riddled with insecurities. That made for a more complex (there's that word again) and richer character than the one of the comic books. The comic book Lois had to fall for Clark when he was strong and respond to him, but had to reject him when he was weak. In L&C, she did fall for Clark but her personal issues caused her to ignore her heart and push him away.
