I agree with you to a point, but as much as Lana *thinks* she knows Lex, and that he's showing her all his sides, she really is still being duped. He's showing her enough, just enough.. to make her feel that he's giving her something that Clark has refused her - trust and honesty. So, in that sense, she is being naive. It's pretty tragic, once again. She thinks she's with this man who will share everything with her, but he never will. I do think that Lex wants Lana's love, and in his way does love her too, but I don't think he's capable of being the kind of man she's going to want to spend her life with. When his true colors flare, and we know they will - she will have a very rude, and likely dangerous, awakening.
This is why she's is being naive. But I think her intentions are good, she's going to give it a shot because she has no reason not to at this point.