He does know just how much of a two-way street this is, doesn’t he?
But what if out of his love, he has been blinded by her true nature and he tells her he's SM and she goes and tells Lex? There's no taking that back. I mean, it's not like he's been blinded by from a woman's true nature before, he's just trying not to have it happen again.
I have to say that I agree that Clark should at least consider the issue of who Lois might tell. I think he is a bit too worried about this, but I guess I also know that Lois can not stand Lex at all, something that he does not know. Also, it is not like Clark has actually not told Lois anything because of this. His not telling Lois is a result of there not being a good time, not because of fear she will run to Luthor.
I think it would be unrealistic if he at least did not weigh this issue. Luthor even getting a hint that CK=SM would be really, really, really bad.
