Sure, he says it's because Lana would like Clark to be at the wedding. Um, if I were Lana, I'm not sure if I'd want my ex-boyfriend whom I'm still in love with to be there.
Then Lex says he wants Clark to see "what he gave up". I'm paraphrasing here as I didn't tape the episode, but it's pretty much what he said. And it struck me: Lex was acting like a jilted lover! Michael Rosenbaum deserves an Emmy for his continual fine performance, because I felt as if he was lamenting the loss of Clark as I slipped my slash glasses on. He was hurt that the "secret that was Clark Kent" was no longer his to unravel as Lana was trying to do, back when "Clark mattered".
Except that Clark still matters. Very much. Clark tried to strangle Lex under the influence of Red K, yet Lex, now his sworn enemy, forgives him all for the sake of Lana? Sure, he could get a perverse pleasure out of seeing Clark at the wedding. Or does he want Clark to see that he let *Lex* get away, not Lana?
As for Clark, he was genuinely apologetic about trying to kill Lex while under Red K. The chemistry between both men was sparking all over the screen. Tom Welling always picks up his game when he acts with Michael.
Of course the Superman canon won't allow them to be friends, but if we throw that out for a second, if Clark had told Lex his Secret only a few years ago, Lex would have held that Secret close, would never have told it to Lionel, would have drawn closer to the Kent family and would have used his wealth and power to keep the Secret safe. We saw a Lex struggling not to become his father. We saw a Lex wanting Clark's friendship and hoping for acceptance by the Kents. We saw a Lex who needed Clark, and a Clark who needed him.
Now Lex can't be trusted with the Secret. And yet, there is always a a part of Clark who will want that friendship back, no matter how deeply buried it is in his psyche, and Lex will want the same, no matter how far he'll travel down the dark path. The loneliness would have been held at bay if they had stayed together.
First loves are never forgotten.