The first time Merlin ends up in bed with Arthur, he looks at Arthur’s hand curled around Morgana’s delicate foot (holding it down, fuck, why is Morgana letting – ), at the precise path of Arthur’s fingers along her ribs as they reach for a nipple (Morgana’s hair tickles Merlin’s chest when her head falls back with a groan, her breasts flushed at the edge of his vision, and – ), and doesn’t look at Arthur’s face, for he (merlinarthurthey) might just lose his temper.
He’s not good at sharing; they move in jolts and half-waves that do nothing to defuse the tension, Merlin’s cock sliding in shuddering jerks along Morgana’s spine every time Arthur fucks into her, and it makes – it makes Merlin feel ill, is what it does, makes Merlin’s insides twist and hurt and curl in on themselves, until it feels so good he comes all over her back.
The second time is, of course, because of Lancelot. Never mind that he’s head over heels for Gwen (still), his adoration of Arthur eclipses everything else: morals, propriety, his loyalty to – to Gwen.
They’re sitting at Arthur’s table drinking wine, while Merlin’s hunting for a particularly stubborn type of imaginary dust. Arthur’s already told him three times he can go, but then he’s sprawled with one leg over an armrest, thighs casually spread, and it’s – positively indecent. Merlin decides he would be a terrible friend if he left Lancelot to Arthur's mercy.
He doesn’t leave when they kiss (solemnly, and Merlin rolls his eyes), nor when they take it further. Arthur scowls at him when he approaches the bed, but Lancelot gives him a warm, hopeful smile, needing a familiar hand to steady him. Arthur huffs, but soon Lancelot’s on all fours, face tucked into Merlin’s neck, moaning hopelessly.
Their cocks are pushed together every time Arthur thrusts in; the mattress is very comfortable, so Merlin relaxes and strokes Lancelot’s hair. Arthur fucks like he fights: tight, controlled and precise. It’s quite boring, really: predictable, except that Arthur’s thighs keep accidentally brushing Merlin’s legs, making Merlin shiver – because he’s ticklish. He kisses Lancelot for something to do, makes it lazy and sloppy; pretends not to notice when Arthur’s rhythm stutters for a moment.
The third time is for a different reason altogether. Arthur’s burning up with fever and shivering with cold, and although Gaius orders a cool cloth to his forehead, Arthur orders Merlin into his bed, curls around him for warmth. Merlin knows he shouldn’t, but Arthur looks miserable, his body hot and shivering against Merlin’s, so he runs a soothing hand over Arthur’s back and holds him close until he falls asleep.
Then Uther dies, Arthur having to step up, and they have entirely different worries. Arthur wants to be a good king, and Merlin helps as best he can, mostly by being a friend. Well, being a powerful warlock helps, too.
The last thing he expects when he opens Arthur’s door is to find Lancelot in his bed, Gwen arching between them.
Merlin feels unreasonably betrayed by this display, or maybe just lonely, but Gwen smiles and reaches out her hand. He goes, a little uncertain now; holds back, just touches Gwen, beautiful, generous Gwen, rubs a nipple and watches her come, jerking helplessly while Arthur’s rhythm doesn’t falter at all.
They’re all finished before he’s even had a chance to touch himself, and he feels self-conscious until Gwen looks at him fondly, wrapping her hand around him. It’s been a long time since anyone touched Merlin like that, and he relishes the moment, smiles back.
Lancelot’s dozing off to their left, and he assumes Arthur’s doing the same, until he hears a soft noise, and cranes his neck. He finds Arthur looking at them, slowly jerking off. Gwen laughs when she notices, practically shoves Merlin at her husband, and after all these years, Arthur moans and pulls him in, fingers clumsy, and Merlin almost can’t believe he hasn’t been imagining the looks Arthur sometimes gives him.
Arthur tries to push Merlin under him, but Merlin’s having none of that; shoves Arthur flat on his back. Arthur’s eyes are wide and a little uncertain, but he lets Merlin spread him wide open; then push into him. Merlin fucks him with no finesse at all, erratic thrusts that Arthur desperately tries to meet, groaning helplessly, fingers leaving marks on Merlin’s hips. Merlin kisses him, open-mouthed and filthy, and when Merlin bites his tongue, Arthur comes all over them.
The fifth time, they don’t quite make it to a bed.