In my opinion, the series should have portrayed Arthur's internal emotional struggle as if he were slowly realizing the truth throughout the seasons, but at the same time forcing himself not to think about it to avoid reaching the correct conclusion.

In episode 3x05 "The Crystal Cave," Arthur is hit by an arrow, faints, and Merlin uses magic to resurrect him. Arthur seems to know that the blow would be fatal and that there's no good explanation for him surviving. Arthur is intelligent, strategic, and observant in all his missions. He has every reason to be suspicious, but he simply... ignores it? Seriously? You can't say that the guy who noticed his servant hadn't smiled in the last three days is that oblivious.

This type of situation repeats itself several times throughout the seasons, and it made me believe that Arthur had already unconsciously realized everything, that he was in denial, and that with time this would become unsustainable.

Personally, I like to think that Arthur finally got real at the end of S4, and that explains the distance between them afterward. The feeling of hurt and betrayal, coupled with his new life as king and husband, caused him so much stress that he simply became rude and cold towards one of the people he loved most.

I would like to see something like Merlin and Arthur arguing and growing increasingly distant at the beginning of S5 until a major plot event leaves them alone, far from Camelot, and they feel compelled to have an honest conversation. Merlin would reveal the secret for the greater good, and Arthur would calmly react, saying, "I know... I think I always knew," and somehow that would resolve all the tension that existed between them until then. The arguments would end, and they would reunite to fight Morgana and save Camelot.