Arthur would very much like to say that it had been a fluke. Nothing more than mere coincidence, that he had done so well in the tournament right after Merlin's mouth accidentally brushed against his in the tent. Merlin is a shoddy manservant, after all, and he doesn't know the proper way to dress Arthur—always stepping too close, leaning in the way he does. Of course they'd eventually end up (kissing) touching faces.
Arthur would very much like to explain this away as a fluke, too: the way he unwillingly seeks Merlin out before the next tourney. Merlin laughs hesitantly for a bit before he bites his lip and pecks Arthur once, flushing and running out of the tent before Arthur can say anything (Arthur takes all of an hour to trounce every participant and it doesn't escape his notice that Merlin's staring slack-jawed at him the entire time).
Arthur has nothing to say about the rest of the times he and Merlin close the flap of the tent and kiss, lightly first, then slow and deep, then with tongues where Arthur never imagined tongues would go, then with eyes closing of their own accord and arms tugging each other close so they can feel each other's presence surrounding them (and each other's hair, and ears, and neck, and).
Arthur's late to every subsequent tourney appearance, but he takes it in his stride and wins every single time as a matter of course. Everyone claps and cheers him raucously, but the bright, proud smile on Merlin's face is something Arthur treasures particularly—despite it usually being Merlin's fault he's late, Merlin having the audacity to possess a mouth that Arthur can't resist sucking and licking into for precious, secret minutes. Merlin's always disconcerted and a bit lost by the time Arthur reluctantly parts from him, and promises him: next time will be different.
It never is, because Merlin can't resist Arthur, either. Arthur saves his memories of Merlin sighing and pulling Arthur in again for particularly vicious bouts against particularly vicious knights, somehow regaining the strength to knock shield and warrior both to the ground.
And when Merlin starts pulling Arthur into alcoves and empty guest chambers and storage rooms right before Arthur's due for anything not involving his manservant? How is Arthur to explain that? Ah, to be fair, Arthur doesn't really spend sleepless nights contemplating it anymore. It doesn't matter to anyone but him and Merlin, does it? He's going to keep (worshipping) utilising Merlin's glorious (everything) lips for the foreseeable future, and Merlin's certainly not complaining about it.
It's all for the sake of Arthur's success, of course. Of course.