Ooooh. I agree with some of what Arawn said and also what Carol said. I actually decided a couple of months ago that I wanted to write a fic about this very thing - the rejection of Clark and acceptance of Luthor and why Lois did what she did. It's been a fun topic to explore.
I feel very strongly that Clark was the only one who could see Luthor for who he really was - and that was only because of his position as Superman. He knew things that no one else could have possibly known.
I can also see how Lois would just think Clark was being jealous when he began attacking Luthor's credibility. He had previously warned her about Luthor in a very cautious, reserved kind of way. But he didn't out and out try to tell Lois that the man was evil or that she was getting in bed with the devil until AFTER Lois had rebuffed his advances. So that made him look like he was just being petty and jealous.
Also, I never felt that Clark was "proposing" to her when he met with her that day in the park to tell her his feelings. He never actually said the words "will you marry me". All he did was tell her that he loved her and that she had to have known. I always felt more like he was saying "Please don't marry Luthor. I love you and I want the chance to get to know you better. If you have any feelings for me, then you'll wait and see if we have anything." I just don't consider that an out and out proposal.
Now, Lois's response for me was not unexpected... and I will also argue that it's not out of character. I think like some of you said, that she was afraid of her feelings for Clark and she was pushing him away. She hadn't come to accept herself that she loved Clark, so how could she possibly admit it to him? What I did think was exceptionally cruel was her asking him to go fetch Superman. But again, I think that was a knee-jerk reaction. I think Clark floored her with his confession and she was afraid and confused and Superman was her port in the storm. Clark did seem to have some kind of connection to Superman. But, while I do think it might have been unorthodox for her to yell "Help Superman" to get his attention, why couldn't she have just yelled "Superman". He would have come even if she had just shouted his name - and then the whole "help" ethics thing wouldn't have been an issue. <shrug> But like I said before, I think the "asking for Superman" was a knee jerk reaction. Clark had just flat out asked if she was in love with Lex, and she stammered over her answer and I think it embarrassed her and sort of threw her for a loop. She knew there was one person she really did love - Superman.
But anyway, what I liked particularly about the whole scenario was that Lois realized she was in love with Clark *before* she knew that Lex was evil. And also that Lois broke things off with Lex *before* the cavalry showed up to stop the wedding.
I think Lois is a very complex character with deep trust *issues* - even with trusting herself. So based on her actions in other episodes (even future episodes in later seasons), I can see her doing exactly what she did.
So there's my take - if it made any sense. <g>
-- DJ
