Well, considering Lois' taste in men - How could she ever fall for Luthor? Or Claude? Scardino certainly would have been the best of the lot! - I don't have any trouble at all seeing her chosing someone else - at least before she knows Clark's secret. And that's not what I'd call 'moving on'.
I don't really know whether she'd move on easily, but the stress is on 'easily' here. I'm sure she'd be able to move on eventually.
The issue is different for Clark. He has eyes only for Lois, so every other relationship would fall under the category 'moving on'. Besides, Clark proves to be a hopeless romantic, and that certainly doesn't make things easier for him. But I'm sure even he could - and would - move on, unless the author gives a very good reason for him not doing so. (As in "She's" and "Time Doesn't Heal", for example.)
