Not to put too fine a point on it, but Sirius is used to being an utter bastard. He's used to throwing who and what he is into the faces of the people he has to live with, because it's his simplest, best, and all too often only weapon.
So I'm a Gryffindor. So I'm a blood traitor. So I've lost more points for my house than Bella ever gained for hers. So I've been arrested by both Muggle authorities and Aurors for things like disturbing the peace. So I tinker with motorbikes. So I decide whether I'm going to be polite by some other criteria than whether or not their ancestors could use a wand. So what?
What he's saying, whether it be at the best of times or the worst of them, is I'm not you, I don't care, leave me alone. He strikes out first, because otherwise he's never going to have a chance to land a blow. He takes their weapons away, tells them he's proud of his most idiotic stunts, because then they can't make him feel bad about it.
Not this, though.
Sirius wouldn't even use this as a weapon against his mother if he could force it past his throat.
He hasn't really been able to force it past his throat to Moony. He has a feeling Remus knows, though. It's in the way Remus leans against him, when they've snuck off to one of the secret passages to talk. In the way Remus is just as reluctant as he is to leave the pile their wrestling matches always end in. In the way they can climb to the roof of a tower, lean against the parapets, and just make each other laugh until James comes looking for them under the invisibility cloak.
They've only really touched or kissed a few times, because Sirius, who was raised with near-Victorian sensibilities, is always awkward initiating it, and Remus is too terrified of pushing it too far, of loosing this and Sirius's friendship in the bargain if he crosses some line. They've never said "boyfriend." They've certainly never said "I love you." Sirius just hopes that he'll manage to spit at least one of them out before they have sex.
But he knows and Remus knows, and if they're too afraid to say it out loud, then that's been all right so far. This is about them, about the kind of bookish kid with the furry little problem as well as Sirius himself, and so this will never, ever be a weapon.
Sirius Black is fifteen years old, the dark mark on his family's otherwise pristine pureblood name, and has enough of a bad boy thing going to give him a shot with half the girls in the school. But he's never really found girls all that exciting, and he hasn't thought of himself as a Black in years. And if there's one thing he hates about himself, it's how his instinct is to lash out at anyone who starts to feel comfortable like family, to take away their weapons and strike the first blow because at some point, they will hit back and they'll hit hard.
But this isn't just about him. He can't make this a weapon and he can't just claim he doesn't care. It's a wide open, gaping vulnerability in what was once a hardened shell.
So it's also a secret.
"Sirius?" Remus asks. "Are you all right?"
Remus presses a little closer. They're shoulder-to-shoulder on top of one of the west towers, looking out over the lake and shivering a little in a mid-October breeze. Remus had been talking about something - some paper, or how he was the only one in the dormitory who seemed to be getting any sleep, lately, something like that. Sirius can't recall what it was just now.
"No reason," Remus answers. "You're just quiet."
Sirius elbows him slightly. "Lot on my mind. You know, James'll be wondering where the hell we are about now."
Remus sighs and leans in a little, until their noses are almost touching. "Well then, if we're going to actually do anything. . . ."
It's an invitation if there ever was one, so Sirius obligingly closes the distance between friends and more than friends. It'll only last a moment, but right now all the so what?s and weaknesses and weapons by which Sirius measures the world don't matter, because right now all he has to think about are lips and tongues and two bodies that haven't quite been pressed together yet, but they're closing that gap, as slow as it's coming. And in a moment they'll part, and roll their eyes at themselves for taking a bloody hour to get there, and be talking and laughing when James or Peter finally try this tower and tell them to come in out of the cold. And in a week or so, it'll happen again.
Because this isn't a weapon, and James isn't a Black, and so Sirius's worst fear is that James won't approve of this.