Merlin knows how to keep a secret.
It turns out that Arthur does, too.
The secret is there in touches that last a little longer than they should, touches that fade away as soon as anyone else is present. It's there in a hundred and one valid reasons for Merlin to be in Arthur's chambers at night, every one of which Gaius believes. It's there in the tiny, crescent moon-shaped indentations on their bodies, hidden beneath layers of clothing. It's there in smiles that turn to sneers when a guard comes around the corner.
Merlin continues to see Gwen and maybe he's leading her on a bit – he does feel bad about it, the same way he feels bad about lying to Gaius so often – but he thinks that it will be worth it in the end. This is his destiny, after all, as the dragon has told him many times (though he'd not considered this interpretation before).
Arthur continues his bizarre dance around Morgana; he seems to have realised that it really was all just playful banter but now he's making an effort to push some tension into the mix too, keep her away from the truth. It's for her own safety, he says.
They change nothing about the way that they act around Uther. Arthur has always been careful around his father, and nothing will change that.
Merlin suspects that Gaius knows, but he doesn't mention it to his mentor – nor, indeed, to Arthur. He doesn't think that Gaius will say anything, even if he does know, unless he feels that someone is in danger.
But Merlin needs to share his secret with someone.
"I think I love him."
The Dragon doesn't reply, only smiles.