"So...this was your idea?" Yamato is completely poleaxed; he'd just assumed this brain-bending proposal was Kakashi's doing, because it seemed exactly the sort of thing his senpai might suggest to get a rise out of him.
Apparently, his assumption was wrong.
"Yes." Sai nods, once. "I thought it would be appropriate; a birthday gift for a lover should be something thoughtful, something that they truly want." Clearly he's paraphrasing from a book he's read.
Yamato shakes his head; he can't be upset with Sai, because Sai is still learning even after all these months together and is bound to make mis-steps every now and then.
So he rounds on Kakashi. "And you didn't—couldn't you have told him—you went along with it?!" He shouldn't be so incredulous, he supposes, considering how well he knows both of them, but still—
Kakashi rolls his shoulders in a minimal shrug, hands tucked in his pants pockets. "It sounded like a good idea to me," he drawls, lazy and warm, and Yamato is floored all over again when he realizes that Kakashi is serious.
"...You...want to sleep with me."
Kakashi shrugs again. "Since always, really."
Yamato feels a sharp stab of irritation that's rooted several years in the past and latches on to it, lets it ground him. "So all that time I was panting after you like a puppy back in ANBU, and you never bothered to let on that you were amenable to the idea of—of—?!"
Kakashi shrugs it off, and Yamato can tell there's a lot more to the story but he knows he'll never hear it. "It was more fun to torture you by pretending I didn't know."
The anger is pointless; the issue has long been irrelevant and Yamato can't really hold on to the ire. All the same, one thing needs saying, and he says it vehemently.
"You were an ass."
"Mm. Still am." Kakashi's tone is completely unrepentant.
Yamato shakes his head. "So then, this is...what? You trying to make amends?"
Kakashi's eye curves up with his smile. "Of course not! 'This' is your birthday present from Sai."
"Let's just say, if we do this now, I'll...have one less regret when I die."
Yamato can't properly digest that; he's not sure exactly what Kakashi means and knows that Kakashi isn't going to explain any better than that even if he asks. So he fires off another question instead. "What about Gai, then? You have to know I'd never go behind his back this way, and it's hard to believe you're willing to."
"Gai...understands. He's given his blessing to this 'most unconventional of youthful unions'."
"But—what does that even—when—"
Sai interrupts his floundering, a faint crease of worry between his eyebrows. "Was I wrong to think this was a good birthday present?"
Yamato's heart flops painfully; he hates having to tell Sai when the boy has completely missed the mark, but he promised Sai in the beginning that he'd never lie. "It's...uh. Well—" He rubs awkwardly at the back of his head. "I...it's just. Things like this are...complicated—"
Kakashi shakes his head and un-slouches, steps closer, right up into Yamato's space. "You always did think too much," he chides, and there's a fondness in his voice that Yamato has never heard there before.
And then Kakashi's hands are reaching up, gently sliding off Yamato's happuri, and Yamato's protests won't dislodge from where they're stuck in his throat. Kakashi hooks a finger in his mask and tugs it down, exposing his nose, mouth, chin; it's not the first time Yamato has seen his senpai's face but his heart is thudding in his chest because they're so close and then Kakashi tilts in to kiss him.
The moment crystallizes in Yamato's head, a frozen split-second of conflicted response; this—Kakashi—it's something he used to want very badly, something he'd made peace with never having years ago. To have it suddenly put before him again, offered and acted on—
He feels Kakashi's lips touch his, feels himself responding to the kiss, and he can't help the shiver that runs through him. He wants it—god help him, he still wants it—but he shouldn't; he has Sai now, who means more than anything, and he can't take the risk of hurting Sai this way, never mind that Sai arranged it—
"I will see you later, then, Yamato," Sai says, brightly, like all is right with the world again, and Yamato tears away from Kakashi.
"Wait—" He can't just let Sai leave, not without understanding how Sai really feels about this, why Sai thought of this in the first place. He loves Sai, desperately, but the boy's brain still works on its own discombobulated plane of operation sometimes and Yamato's got to talk to him—
But it's Kakashi's hand that closes lightly around Sai's wrist to stop him as he turns to go, Kakashi who draws Sai back. "Where are you going, Sai?"
"...Somewhere else," Sai says, clearly finding the answer obvious but responding dutifully all the same.
"To give you privacy." Sai's puzzlement is acute, and Yamato shares it. "I thought it would be appropriate?"
Kakashi shakes his head. "I think Yamato will be more comfortable if you stay," he says, conversationally.
The situation is much too far beyond Yamato's experience or expectations for anything about it to qualify as 'comfortable', and he can't even find the words to try to start sorting it out.
"...You're suggesting voyeurism?" Sai asks, somewhat tentatively, but Kakashi shakes his head.
"It's like this: Yamato may want me, but he also wants you. He wants you more, and for completely different reasons. But since this is his birthday present, shouldn't he have both of us?"
"Oh," says Sai, understanding of a sort smoothing over his features, and Yamato envies his easy acceptance.
"I'm not getting in between you two," Kakashi says, and his bared face shows just enough hint of expression to belie the lazy disinterest in his tone. "Besides—" he shrugs, and his gaze flicks down Sai's body and back up with obvious appreciation that makes Yamato's heart skip a very unexpected beat "—I'd be lying if I said you don't interest me a little." He smiles, a familiar intimately-unbalancing smile that is three times more effective now that his mask is down. "If we're going to do this, it should be all of us."
"Wait, wait. Just—hold on a second." Yamato finally manages to speak up. "Kakashi-senpai...just—hang on. Excuse us—Sai, I need to talk to you, please."
Kakashi gives a negligent little wave of his hand. "Take your time, take your time." He shoos them toward their bedroom. "I'll be right out here; just let me know what you decide, hm?"
Yamato snatches up Sai's hand and nearly drags him to the bedroom, shuts the door firmly behind them and turns, resting both hands on Sai's shoulders as Sai peers up at him, worry faint but plain on his face.
"Sai. I—" He draws a deep breath, trying to figure out what to say, where to start.
"You're upset," Sai says, and the set of his mouth shifts enough to add a measure of sadness to the worry in his face.
"No, no," Yamato hastens to assure him, squeezing Sai's shoulders briefly. "Surprised. Really surprised. And maybe confused. This is very unexpected."
Sai's expression shifts again, questioning now, and Yamato plows ahead through his scattered thoughts.
"It's...just. I'm not—not looking to...to—" He's never been sexually adventurous, is really very garden-variety vanilla about most things when it comes down to it. He doesn't need kink and excitement and surprise and variation; he likes knowing that he's got someone to come home to who means the world to him, who's just as fond of him, and that's always been enough. "I'm satisfied with you, Sai. With you, just as we are. You don't have to...to try to 'spice things up' to keep me interested."
"Okay," Sai agrees, looking mildly puzzled, which is enough to tell Yamato that spicing things up hadn't even crossed Sai's mind and he's still pretty far from the mark. He heaves a small sigh, gathering his frazzled wits with the slow exhalation, and tries again.
"Tell me why you decided on making Kakashi my birthday present," he says, gently. "It's...very unconventional, and I need to understand where you're coming from."
Sai frowns thoughtfully. "It's obvious that both of you care about each other, are attached to each other, more than a shared history of comrades-in-arms usually warrants. There is obvious sexual attraction. It's not clear whether you're suited to a romantic relationship with one another in the long term. You...idolize him, I think is what I mean to say, and Kakashi is extremely...guarded, about the bonds he allows himself to form with others. It's unclear whether you could have developed a successful intimate relationship, but it is clear that the attempt should have been made."
Yamato is taken aback—not so much at Sai's brutal honesty, which he's grown fairly accustomed to, but at how Sai can be so astute and perceptive about relationships and emotions between other people but still have so much trouble with his own.
"Perhaps it's irrelevant now, as both of you have established intimate bonds with others. But there is tension that remains," Sai continues. "It's...like a question asked between you that both of you ignore. You should have had sex with each other years ago, even if it would not have culminated in a relationship. You need to have sex now; it's clear that both of you still want to even though Kakashi-senpai is with Gai-san and you are with me. I believe it's called 'closure'."
"...He wants you to be involved, too," Yamato finally says, because Sai is right and he can't think of anything else to add. "How do you feel about that?
"Perhaps I am to be a 'safety mechanism'," Sai returns thoughtfully. "My participation might serve to prevent the encounter from becoming too intimate for him. I think that Kakashi-senpai is a man who regrets very deeply; my presence could be intended to ground him in the the present and curb his regret at not pursuing his attraction to you in the past."
Again, Yamato is impressed by Sai's insight. "I don't think you're wrong about that, any of that. But do you even want to? Be involved, I mean?"
Sai considers for another moment. "He is attractive, and renowned, and his interest is very complimentary. I think I should feel...flattered?"
"Most people would, yes." God knows Yamato would have been extremely flattered if Kakashi had ever betrayed an interest in him back in ANBU.
"But that is not the same as being attracted to him. I've never given him any thought, sexually; considering it now, I...don't think it would be unpleasant."
Yamato only nods, a slight motion; he can see that Sai is still working the original question through in his mind.
"...Would you enjoy having us both?" Sai finally asks, a faint touch of something that might just as easily be curiosity as insecurity in his voice, and Yamato pauses.
Kakashi, and Sai, both in his bed—it's such an outlandish idea, but he can't deny that considering it has made him hard. "I...well...yes. I think."
"Then I want to do it," Sai says, simply, easily, and his smile is demure and sincere.
And that, apparently, is all it takes to make up Yamato's mind. "Alright," he says, equal parts surrender and acceptance, and dips down, presses a lingering kiss to Sai's beautiful smile. He's feeling giddy anticipation and rational trepidation and a whole slew of other emotions too numerous to name that have him balancing a fine line between excitement and nausea, but he won't let them hinder him now.
The decision is made.
All that's left to do is forge ahead with it.