Actually I think it is a bit of a assumption on Clatk's that Mason couldn't accept superman if she really understood who he is.
I think Clark makes the assumption because he isn't interested in Mayson enough to give her the benefit of the doubt.
Coming across as quite the Mason fan, which I'm not really, but every time I watch the middle of session 2 it annoys me how Clark strings her along and doesn't put her out of her misery. He clearly never wants to go on any of the dates she invites him on.
Having been in Mayson's shoes more that once in real life I can't but wholeheartedly agree.
And while I enjoy the romance between Lois and Clark, I feel for the poor lawyer every time I watch those episodes...
