I imagine that the Marauders (and Lily and Marleen and Mary and… well, all of those arseholes, really) tell each other that they love each other all the time. It starts out in their very first year, with James telling them all he loves them because he was always a very affectionate child and the others latching onto it because none of them got enough love in their childhoods. (Remus is a fucking werewolf, Sirius came from the Black Household, and Peter’s sister got all the love, since their mother never wanted a son; she’d only wanted a daughter.)
They’re all doing it by the end of First Year. It just becomes a part of their vernacular.
Naturally, Sirius and Remus say it to each other just as much as the others, but Sirius can’t –for the life of him, damn it– figure out if it means anything else. He wants it to –boy, does he want it to– but he has absolutely no idea how to distinguish between the platonic and… well, the less platonic. So he says nothing.
And then, they start fucking.
That’s all it is. Sirius pushes his emotions down because he can’t think about it any other way. Remus is just a mate helping a mate. They’re just… doing it to do it. Practicing for the girls. It doesn’t matter that Remus sleepily mumbles that he loves him as they cuddle after sex, because that’s what they’ve always said. Nothing has changed. The gaping black hole in his heart doesn’t matter.
It isn’t until Remus comes up to him in the Common Room and curls up in his lap that Sirius actually questions it. He stiffens minutely, but it’s enough for Remus to be like ??? What’s wrong??? You literally had my dick in you last night are you okay??? I just kissed your neck Padfoot you look like you’re terrified I’m going to bite you.
And Sirius stares at him for a full thirty seconds, in which Remus begins to squirm a little under the gaze, and then blurts out YOU LOVE ME???????
And Remus’ eyebrows skyrocket. They fuckin’ leave the planet’s orbit. “Yes??? I tell you that a lot???”
“But you tell Prongs you love him. You tell Wormtail you love him.”
Remus stares at him and then slowly, slowly begins to shake his head, because it had never even occurred to him that Sirius might doubt his actual intentions. He supposed they did sorta skip the whole Talk that usually came with dating, but he’d assumed it was implied. It had been implied!!!
But apparently, Sirius hadn’t picked up on that particular implication, because he’s still staring wide-eyed down at Remus. So Remus sits up properly and kisses him, long and full on the mouth. Sirius melts into the kiss, like he always does, because he can’t resist because Moony is so brilliant. When Remus finally pulls away, Sirius’ eyes are slightly unfocused.
“I don’t do that to Prongs,” he points out, and nuzzles his nose back against Sirius’ collarbone. “I love you, Sirius. I want to date you. To be honest, I kinda assumed we were already dating.”
And Sirius just kinda goes……. oh.
It’s worth Remus’ soft puffs of laughter against his skin, though. His Moony is so brilliant.