There was more than that one date. There were lunches and the episode where they are taken hostage at the Planet they were on their way to see Madame Butterfly I believe. It is also mentioned between Clark and Lois that she is seeing Lex.
I've wrote earlier what I felt about most of this so I won't rehash it. However I will say that I believe the reason Lois excepted the proposal was the rejection by Superman. Whether or not she was truly in love with Superman she had convinced herself that she was. That can make the pain of rejection a lot worse that if her feelings had been geniune. I'm not saying she didn't love Superman, I'm just saying that we can build things up in our own minds to be a lot more than what they really are. So when Superman rejected her and in the fashion that he did that she turned to Lex to boost her ego etc. The way it look in the episode was that she had gone to see him that same night. So she jumped in as a knee jerk reaction to the rejection. From then on in the story you see her having second thoughts. This is evident in the discussion on the house Lex is building, her meeting Mrs. Cox, even in some of her dealings with Clark. But Lois doesn't admit she's wrong. In this case she almost waited to long.
