Xanxus can—just barely, and always with the sense that he is somehow getting away with something—get his head around Squalo and the fact that Squalo is his. It took about a decade to do it, plus about three feet of bone-pale hair, but yeah, he gets it. Squalo is his right hand and, (this is how Squalo puts it when he talks about it late at night, half-drunk and probably convinced Xanxus isn't actually listening to him) Xanxus' sword, too.
Xanxus still doesn't know what he thinks about that, so he mostly doesn't—just lets it be what it is and focuses on other things instead.
What he can't wrap his brain around is this: Yamamoto Takeshi with his legs wrapped around Xanxus' waist and his hands locked on Xanxus' shoulders, groaning enthusiastically as Xanxus fucks him. This happens sometimes. Hell, it's starting to be a semi-regular occurrence, and the weirdest part of it all—aside from the fact that he's pounding into Yamamoto and Yamamoto just gasps for more and harder and yes fuck right there as his back comes off the wall and his body wrings down on Xanxus' cock—the weirdest part of it all is that Xanxus still doesn't know how this keeps happening. Or why.
He gets that Yamamoto and Squalo have a thing of their own, one that goes back to the days when Yamamoto would trail Squalo around like some kind of katana-wielding duckling who'd imprinted on an inappropriate parental object. It had involved him calling Squalo senpai and Squalo doing a lot of frustrated screaming as he kicked Yamamoto's ass in their sparring sessions. Somewhere in there, Yamamoto turned into a credible swordsman and Squalo conceded that the kid wasn't a complete waste of oxygen and now Xanxus sometimes walks in on sparring sessions that have turned into something else altogether. Apparently this is a thing that sometimes happens between the ones who are sword crazy.
Xanxus doesn't get it, but the view's generally pretty nice and Squalo always comes back to him once he and Yamamoto have gotten it out of their systems, so he guesses it doesn't matter.
But sometimes Yamamoto shows up when Squalo isn't around. Xanxus thinks the first couple of times were accidental. They were almost certainly accidental. Yeah. He's still not entirely certain, even now, how Yamamoto manages to time his visits just to when Xanxus is in the mood to fight, or how Yamamoto manages to get them from fighting to fucking. Maybe it has to do with the number of times he's seen Yamamoto drop to his knees to suck Squalo's cock after they're done with the swords, or walked in on them while Squalo is hissing and swearing as Yamamoto's cock slides in and out of his ass. Kind of seems like that's what Yamamoto assumes the natural end of any fight ought to be, anyway, and since he learned it from Squalo in the first place, or Squalo learned it from him and transmitted it to Xanxus, or something, it probably all makes sense somewhere.
Yamamoto groans again as Xanxus hitches his hips higher and fucks him faster; he's got a grin on his face and he runs his fingers along the nape of Xanxus' neck, just the edge of his nails against Xanxus' skin. That hint of sharpness pulls Xanxus over the edge; when he's done gasping and shaking with the heat that rakes him open, Yamamoto's fingers are curled around his nape.
"The fuck," Xanxus says afterwards as he's catching his breath, loath as he is to admit that Yamamoto has him baffled. "The fuck are we even doing?"
Yamamoto just grins at him. "I think the daishō suits you," he says, and doesn't bother to explain what that's supposed to mean.
Xanxus rolls his eyes and chalks it up to another one of Yamamoto's eccentricities. It's not like Yamamoto doesn't have enough of those to go around.
He doesn't get it until months later, when Squalo is recounting his fight with a would-be swordsman who, he says disgustedly, hadn't even deserved to look at a sword, much less wear daishō. Oh, Xanxus thinks as Squalo tells him that that's a set of paired blades, though he doesn't know what to make of that now that that's been cleared up.
But maybe he doesn't have to make anything of it. Maybe it's just enough to let that be what it is, too, for however long it lasts.