Even if the rest of the Leaf doesn't understand, Naruto and Sakura know that Sasuke isn't gone...he's just beneath. [Rated for dark sex]
They see a triumphant Hokage pinning down the woman he loves, the woman he's always loved more than anything in the world, on the grave of the man he hated, and making passionate love to her. To both of them, it is a way of spitting at the memory of someone they hated.
It's rather disturbing, true, but after all the two of them have been through together, after all they've gone through to help the village, they're allowed a few unsettling quirks.
Normally, Sakura straddles him in bed, pinning him down, letting her hair fall down around her face and into his eyes like a curtain that closes the two of them off from the world.
(She told him once that she wanted to lop off all the rest of her hair too, leave herself feeling free and light-headed, but he begged her not to. He said he wanted her to stay the way she was when he woke up to see her defending him and Sasuke in the Forest of Death.)
When they come to Sasuke's grave, it's different.
"I want to be as close to Sasuke-kun as possible," Sakura explains to Naruto as she strips off her clothes.
He nods, because he understands; he always understands. Sometimes she feels that maybe she should let him take the bottom for once, because he must want to be close to Sasuke too, but she's still a bit selfish. Neither of them has grown up entirely.
When they're both naked she grabs him by the hips and yanks him onto her, pulling the two of them down to the ground. They needn't worry about hitting their heads on a tombstone. Sasuke's grave is marked only by a small patch of granite set in the ground. His name is not on it--only the characters for /Uchiha/.
Naruto squirms against her for a moment, and then he enters her with a wrench of breath.
Sakura jerks back into the earth, and suddenly it's a comfort. It's cool and soft and somewhere beneath it all is Sasuke, waiting for her. She grabs Naruto and pulls him as close to her as she can. He's done so much for her--the least she can do is share Sasuke and the earth with him now.
The images start a little while after that, pressing into her from below as Naruto presses into her from above. The cracks inside her head fill up with Sasuke, just like she always wanted. She can see Sasuke stumbling out of a cave to greet her; Sasuke being pulled from the depths of a lake to embrace her; Sasuke falling out of a dark cage into her arms--
--Sasuke stretching out in ground below Naruto and Sakura and feeling his way up to them.
(Sakura is a medic. She once explained to a patient that in the heat of excitement, the desperate and the unstable can conjure up hallucinations. They often start out as tactile sensations.)
It's then that she feels Sasuke curling himself around her. He nuzzles the curve of her shoulders and neck, and he puts his arms around her and rests his hands on her stomach. As she begins to rock in his embrace, he slides his hands down until his fingers are tangled up in the heat of her, shifting and fluttering with a skill that in life, he never had a chance to use for anything more than killing.
"Sasuke-kun," she pleads with him. "Sasuke-kun, stay."
(When her mouth opens and closes like this at home in bed, Naruto fills it with kisses. Here he would not take that privilege from Sasuke.)
She struggles to sink deeper into the ground, closer to Sasuke, as Naruto holds her to the earth. Her eyes blurring with the tears she won't shed, she turns her head and she seeks his mouth with hers. "/Sasuke-kun I need you right now--/"
And then she's not sure whether she feels his lips brush hers or not, because that moment is always when the ache uncoils inside her and she comes. Sasuke retreats back into his grave even more rapidly than he arose after that.
What's next is the worst part: the time between when she comes and Naruto does.
(The first time they did this, she cried hysterically at that point. It made Naruto frantic, and he asked her if he should stop. She nearly screamed at him not to stop. He had to finish.)
In that interval, Sasuke is trapped in the ground beneath them, but she can sense him reaching for them, hear him begging to be released. His voice is muffled, but Sakura could swear that he's saying something about love, and he's trying not to sob.
Finally, Naruto yells out and sinks against her. After a moment, he catches his breath, and he lifts himself enough to lean past her shoulder and press his face to the cool granite of the Uchiha grave marker.
"Sasuke," he whispers. He gives their teammate back his name, and as his lips move, they brush the earth.
And Sasuke is silent until the next time.
It is a strange custom, to be sure. The village knows that. But regardless of his funny little quirks, like eating too much ramen and having sex with his wife on their enemy's grave, he is the best Hokage ever, and the population of the Leaf is grateful for that. He was plagued by the cruelest childhood anyone could imagine, by the horrors of being an outcast /jinchuuriki/, and he overcame it all while still being a kind and wonderful human being. He was spurned by the love of his life in favor of a cruel monster, but he overcame even this adversity to win her love.
It is the best possible ending to an amazing story, and whenever those who know them see Uzumaki Naruto and his Sakura together, they can tell that the two of them are perfect and invincible.