If Clark Kent/Superman hadn't been there do I believe Lex Luthor would have proposed. No. Lex was incapable of loving any one. He was to in love with himself for that. Lois was beautiful, smart, talented, etc. That is not what made her desirable to Lex. It was her independence. She didn't throw herself at him like other women. She was a challenge. That is what made her desirable. So no he would not have sought marriage he would have sought to bed her.
The reasons as follows are why I don't believe a proposal would have been forthcoming:
1. If Clark wasn't at the ball to cut in who knows how things might have progressed that night.
2. Even after being asked by Clark how far she would go for a story she goes to Lex's home for dinner dressed more as if for a date than for an interview. She stays once she sees the romantic setting, she flirts with him at dinner, she allows him to escort her home and to her door, she allows this kiss on the check. That was not a business dinner that was a date. She never set him straight about that being all she was interested in. Then she continues to see him. Without Clark there to continually question her relationship with Lex who knows how far she would have gone to get the interview.
3. Superman in the original universe shows up and becomes a double challenge. a) To Lex's power and empire. b) To Lex's ego - which in my book is the more dangerous. He is an egoistical psychopath. To me when Lois' very obvious attraction for Superman became evident to Lex (the pheromone kiss at the airport and the bachelor auction) a part of Lex's rational judgment left him for good. He would do whatever it took from then on out to win Lois from Superman. It wasn't so much about him really wanting her as it was not letting Superman win her. His ego was at stake - he always won and he wasn't about to start loosing now especially to an alien. He would do whatever it took to win.
4. The only threat Clark Kent was in the original universe was sort of being Lois' conscience and a haven to run to. That Lois could actually want Kent was inconceivable to Lex. He saw Clark as being beneath himself and Lois.
5. Lois' independent nature was the the real attraction. If she still hadn't succumb (without Clark there to be her conscience) Lex would have done what ever it took to break her. a) Getting her drunk and seducing her. b)GHB and rape c) kidnapping and holding her prisoner to slowly break her will to make her come willingly to his bed. He believed for him there were no consequences he couldn't use his money to make go away.
So no without Clark/Superman there he wouldn't have proposed. In the original story the only reason I believe he really tried to kill Superman is he saw Lois waivering in her decision to marry him. As long as Superman and Clark were a live she still had 2 people to run to as a haven. But Superman had to know his defeat that was why he didn't kill him outright. Let him lay there dying listening to Lex's victory - the marriage ceremony. Then when Lex was through with Lois he would have disposed of her by what ever means were most convenient.
