In retrospect, Gokudera should have suspected something ridiculous.
Yamamoto has been entirely unsuspicious all day, engrossed in the latest manga he's been reading and apparently content to stretch out under the kotatsu and stay there for hours without moving. Gokudera would have complained at the other boy taking up all the space, but as it turns out Yamamoto's legs fit remarkably well under his knees, and the other boy's radiant heat is more than tolerable given the winter chill of the air. From where he's sitting Gokudera can glance over and see the inch of bare skin at the back of Yamamoto's neck, just between his collar and his hairline, and with Yamamoto lying on his stomach so he can hover over his book there's no chance Gokudera will get caught with his gaze lingering. Yamamoto has a basket of cookies in front of him, has been eating them with the slow steady pace that promises devastation to the food in front of him, and Gokudera has a half-finished mandarin, the peel carefully stripped off to leave the slices for his consideration. It's pleasant, actually, the warmth of the kotatsu and of Yamamoto's jeans against Gokudera's legs, and Gokudera, for once, isn't thinking about anything at all other than idle contemplation of his current contentment, wondering if maybe this is how Yamamoto feels all the time.
"Hey, Gokudera."
Yamamoto isn't speaking that loudly, just his usual clear enunciation, but in the unusual peace of Gokudera's thoughts the sound is startling. It makes him jump, flushes him hot with a rush of unnecessary adrenaline, and when he looks over at the back of Yamamoto's head it's with a frown the other doesn't see.
"What?" he snaps.
Yamamoto doesn't look back. To all appearances he's still engrossed in the page in front of him; Gokudera is more than half-expecting some question about an obscure reference in the plot. He is definitely not expecting what he hears, which is Yamamoto saying, "We should get married."
Gokudera's mouth comes open, trying to form itself around some sufficiently aggressive response that fails to materialize on his lips. There's no question about his reply - it's just finding adequate levels of no, deciding where exactly to start with this, that is tripping him up.
"No," he finally manages, sticking with the straightforward route. "No, no we should not, what the hell is wrong with you?"
"You don't think so?" Yamamoto looks back over his shoulder. He's still holding a half-eaten cookie, eyes wide and curious like he really doesn't understand Gokudera's response.
"Of course not," Gokudera snaps. He angles his leg, kicks at the back of Yamamoto's leg so the other laughs a halfhearted "Ow" as Gokudera clarifies. "We're in high school, we can't get married." He tears off a slice of mandarin, eats it with somewhat more aggression than is needed, and Yamamoto is still blinking at him as if he hasn't considered this. "And you can't just casually say something like that, you have to do all the traditional stuff too."
"Like what?"
Gokudera stares at Yamamoto, trying to decide if he's being teased or not. If so Yamamoto is a much better actor than Gokudera has ever considered him to be; there's no trace of mockery in his eyes or in the soft unconcern at his mouth. He appears to just be listening with complete attention, absorbing information like he sometimes does when Gokudera tries to help him with homework.
"Shouldn't you know this part?" Gokudera finally demands, pulling another slice free to throw at Yamamoto's head. It's supposed to hit him but the other's reflexes save him, get his hand up in time to catch the makeshift projectile so he can eat it instead. "Idiot. Everyone knows this, you need a ring and you kneel down and you're supposed to ask, stupid, not just tell." He kicks at Yamamoto again, his ankle, this time, and Yamamoto grins and pulls his legs up so he can press his feet against Gokudera's and cut off the other's motion. Gokudera stops kicking but he maintains his glare, takes the next slice of mandarin for himself.
"And we're in high school," he repeats, the words vicious over his lips. "No one gets married before graduation. And who said I want to marry you anyway?" He looks down at the star-shaped peel spread out under the few remaining slices of the fruit. "We might not even be together in a few years."
"We will be," Yamamoto says, sounding so utterly certain of himself that Gokudera's stomach drops like he's in freefall for a moment.
"Don't be an idiot," Gokudera insists, throws another slice for Yamamoto to catch. "You don't know that."
"Yeah I do," Yamamoto insists, with the gentle self-assurance that is no less absolute just because it sounds so casual. He's grinning again, the soft curve of his mouth that always derails Gokudera's thoughts more effectively than argument, and the only thing Gokudera can do is to offer a huff of skepticism before he reaches out to push the last slice of the mandarin past Yamamoto's lips himself.
He wishes he could be as certain as Yamamoto is. Still, it becomes easier to believe him every time he sounds so secure in their future together, like his belief is catching contagious into Gokudera's mind too, and Gokudera doesn't even want to resist anymore.
He supposes that's a kind of security in itself.