There's a scene in Season 10's "Ambush" that I really like where Lois finds her sister, Lucy, kissing Clark. Lois' reaction is to drag Lucy away and tell her to go crush some ice. Lois doesn't doubt for a moment that Clark is faithful to her and Clark says Lucy surprised him. But then they have an argument anyway.
CLARK: "You know what's a bigger surprise? Is that you couldn't stand up to your dad. Unless you actually agree with his vigilante registration act?"
LOIS: "Clark, I am just trying to get through this 'holiday' without a fight.
CLARK: "Lois you either agree with him or you don't."
LOIS: "I don't. But this means something to the General so please do not make me choose sides."
CLARK: "I thought you were somebody who stood up for what she believed in. And then when it comes to your father, you do the same thing that he does; you make your choices based on fear."
LOIS: "Hey just because you don't have a -- "
Lois stops talking.
CLARK: " ... a what, Lois? What were you going to say? A father?"
LOIS: " ... okay. What I am trying to say is that you haven't had to deal with a family for a really long time. All you have to answer to is yourself. Clark, if you want be with me, you're going to have to deal with my family too. I hope you can respect that."
I thought it was a really well-written episode in a well-written relationship that really showed how Lois and Clark were both devoted to each other but still had the capacity to oppose each other in various conflicts.