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It starts during one occasions where all four of them are together. They're still trying to figure out how to incorporate a fourth person, because it's not exactly simple to even fit them all into bed, let alone make sure everyone has fun and doesn't end up on the floor with another elbow-induced black eye. Fortunately David is a remarkably good sport, enjoys watching, and is open to a fair amount of experimentation. He has an interesting tendency to observer Evan, John, and Cam for a while and then, at some later point, inform them that he has an idea about how if they do it this way instead of that way all four of them could get off at once.
So it's not a complete surprise when he muses, "You know, I think I'd like to try that."
Evan and Cam stop what they're doing and look over at David, who's sprawled languidly on the couch with his dick laying hard on his stomach, the very image of debauchery. Cam, ever the gentleman, offers, "You and I can switch places."
"Actually, I meant in John's position," David says. "Not right now, I don't want to interrupt."
That's for the best, really, because Evan and Cam are both balls-deep in John, and from the way he's growling Evan's pretty sure that if they didn't start to move now John would kill them all. They go back to fucking him, leaving David's idea for another day.
It takes a while to actually do it, of course, because it's Atlantis and there's some kind of crisis every day, be it big or small, which makes getting them all in one place difficult. It doesn't help that they try to be discrete; by this point half the city probably knows about either Evan and David or John and Cam, but that doesn't mean anyone needs to know about their weird polyamorous foursome thing. That particular night it ends up being Evan, David, and Cam together, which isn't quite how Evan would have preferred it owing to how Cam is hung like a horse, but John's off-world until late and David's been driving Evan insane every night with his speculation as to how it'll go. Besides, the two of them have plenty of practice.
Evan preps David while Cam lies down beside them to watch. He starts with fingers first, enjoying the feel of David opening up for him just as much as he did the first time, and then moves on to one of their toys. It's one the largest that he has smuggled into the city over the years, a rippling silicone probe that gradually widens to almost two inches thick. Evan takes his time to work it all the way in and gently fucks David with it for a while. He knows that David doesn't get off from taking it rough as much as John does, and he also feels safe in the knowledge that no matter how long he spends using the toy, David isn't likely to come from anal stimulation alone, especially not when Evan's taking care not to hit his prostate.
After a good fifteen minutes David's lying there utterly loose and relaxed, limp everywhere but his dick. He's breathing deep and smoothly, his eyes are shut, he has a contented smile on his face, and there's a blush creeping down his neck. Evan's about to suggest they move along, but Cam reads his mind and beats him to the punch.
"Y'know, I think he's about ready," Cam says, his voice low and filled with anticipation.
"'ve been ready," David murmurs, not even deigning to open his eyes. That changes when Evan alters the angle of the probe just a little and presses, drawing a squirm and a gasp.
"Well, if you're sure," Evan says, drawing the toy out and watching as David squirmed some more at the sudden empty feeling. He sits back himself and lets David decide where to go next.
"I think," David says after a moment, "that you should lie down, Evan. And Cam, if you'd just?" He makes a little motion with his hand and Cam complies, crawling down to the foot of the bed. Evan lies down in the center of the mattress and David quickly straddles him. Evan takes a moment to lube himself up before tossing the bottle on down to Cam and waiting.
Then David reaches down, lifts Evan's cock up and holds it steady, and slowly sinks onto him. Evan slips right inside without so much a pause and for a moment he just closes his eyes and savors the slick heat around him. David leans forward until he's supporting himself on his forearms, his body just inches from Evan and radiating warmth, and he draws Evan into a long, sloppy kiss. They break for air and David glances over his shoulder and gives Cam a little nod.
Cam gets up behind them, nudging David to spread his legs just a little wider and give him more room. For a long moment nothing happens, then Evan feels Cam slide one thick finger up inside David. Suddenly his ass isn't feeling nearly so loose but it's apparently good enough for Cam, because the finger's withdrawn and almost immediately Evan feels the head of Cam's dick nudge its way inside. David grunts deep in his chest and his breath catches. Cam pauses for a second then starts to slowly press further inside, inch by inch, his cock sliding right along Evan's, until he's buried all the way and his balls are resting against David's ass.
David is impossibly tight now and his entire body's trembling. His eyes are squeezed shut and he's not breathing, or making any noises at all for that matter.
"You okay?" Evan manages to say, his mouth dry and the words hard to concentrate.
David opens his eyes and looks right into Evan's, the biggest grin Evan's ever seen appearing on his face. He nods and takes a deep, sudden breath.
"Oh, yes, I'm doing great," he manages to rasp out. He licks his lips. "I think you know what to do from here, gentlemen."
Cam laughs, sending vibrations through their intertwined bodies, and he gives David's thigh a fond stroke. "You know what to do, Evan?"
"I'm not sure," Evan says, having to stop and swallow as David shifts a little. "I'm at a bit of a loss, really."
"Same here," Cam says. Evan can hear the strain in his voice, knows exactly how hard it is to stay still.
"Guys," David gasps, eyes half-lidded. "That means fuck me."
"That we can do," Evan says.
He doesn't have a lot of room to move with how they're positioned, but it's enough to roll his hips down and back up. Cam follows suit with a longer stroke of his own, a smooth pull out and push in. It takes a couple minutes for them to get it right, to figure out when and how to move and adjust to this being David instead of John, but soon they've got themselves a rhythm set up. Evan's moving in small little rocks that make him rub against David's prostate just right; Cam's going for long, smooth thrusts that have his balls slapping against Evan's and David's ass. For a while there all three of them are lost in the moment, their world reduced to the slick slide of cock against cock and skin against skin, even David at a complete loss for words and reduced to grunts and needy whimpers.
They're close, Evan knows, but even this isn't quite enough for David as much as he probably wishes it was. His biceps are shaking from the effort to keep himself upright and he's desperately trying to rub his heavy cock against Evan's stomach, leaving trails of wetness behind. Evan reaches between them and strokes him, once, twice, and that's all it takes before David cries out and spills all over Evan.
That's it for Evan and Cam, too, because David squeezes all around them. They manage a last few hard ruts and then they're blowing their loads inside him, their come mixing together around their cocks. Evan suddenly finds himself with an armload of scientist and a moment later Cam slumps over them as well. They stay that way for a while, their chests heaving against each other and their dicks slowly softening until they eventually slip out of David. Finally they roll apart, but none of them can be bothered to get up or give themselves more than the most cursory wipe-down. They end up snuggled close together, with David curled between Evan and Cam's warm bulk.
At some point in the night Evan is dimly aware of the door out in the main room opening and closing. John's not supposed to come here tonight, but then, discretion's never stopped him from doing anything before.
"You guys stink," John mutters from beside the bed. There's a trace of laughter there as he speaks. The mattress dips as he settles down on the far side of Cam. "Have fun?"
"Damn straight," Cam drawls softly. "Too bad you weren't here. We outdid ourselves."
"Always next time," John says.
In a sleepy mumble, Evan offers, "I'll blow you in the morning."
John chuckles. "I'll hold you to that." There's a fleeting caress, someone reaching over and running fingers through Evan's hair. "Night, guys."
"G'night," Evan replies. David grumbles incoherently and shifts a little in Evan's arms. Evan pulls him tighter before drifting back to sleep himself.