Between the Pillows and Blankets
Those who knew about them always figured that Merlin was the small, gentle, fragile one – the "girl" of the relationship. After all, Arthur was a warrior, strong, imposing, the King, while Merlin was always stumbling on his own feet and blushing if anyone even mentioned anything remotely sexual. The older knights, those who had been loyal to Uther, used to joke that one day Arthur found Merlin with Gwen's clothes and confused him for a pretty new maid – and ended up enjoying it (all of them agreed that Merlin's mouth was a temptation, so full, and red, and soft and sexy in a very innocent way, it would tempt anyone, regardless of how much one enjoyed women).
And, sometimes, Merlin really was all of those things. On other days, he ordered the King to kneel, demanded touches and caresses and dominated completely the one that commanded the lives of the whole kingdom. But that was just sex, and sex is always related to power (especially when it is between two men), and deep down, it wasn't important. The sex was just a part, a game they played, but it wasn't the only reason they were together – there were other things, loyalty, understanding, love - things that other people rather not see, for they couldn't accept that it was anything but an unstoppable desire.
They didn't see that Arthur and Merlin shared a bed every night, even after the desire was spent. They didn't see how Arthur held Merlin from behind, smelling his neck before sleeping, and they didn't see how, when Arthur let go of him and turned to the other side, Merlin held him, throwing his leg around the king's hips. They could even imagine that, some nights, Merlin laid with is head on Arthur's chest and slept in peace while the strong arm held his thin body, but they knew nothing about all those other nights where Merlin became a pillow in which Arthur laid, complaining about how bony his servant was.
They didn't see that, some nights, Arthur came to bed with his back aching and Merlin massaged them, and as Arthur slept on his belly, Merlin slept with his head over his master's back. They also couldn't see that even when they were turned to different sides of the bed, they kept touching each other all the time, their finger intertwined, their backs aligned against each other, and their warmth under the blankets.
They could see the joking and teasing, the looks full of desire and the body reactions that couldn't be hidden, but they didn't see the slow smiles and long looks, the night talks and confessions against the light of the fireplace. They didn't see they were two sides of the same coin – they saw what they wanted to see, and it was alright.
Arthur and Merlin didn't need any witnesses save for their pillows and blankets.