Kirishima lost a dumb bet with Kaminari and Sero and had to steal stuff out of Bakugou's room to hide them in his own for two weeks. He was not allowed to get caught, so every time they studied in Bakugou's room, Kirishima would snatch a shirt or whatever and leaves. But a few days later Kirishima accidentally wore one of Bakugou's shirts. Oops. BBQ for the Bakusquad paid by Kirishima.
When Kaminari and Sero tell Kirishima to steal things from Bakugou's room, he starts with simple stuff like blank paper, and they tell him it's too easy and he has to steal something personal, like clothes!
That's when Kirishima knows he's going to win, because Bakugou and he already share clothes, like true bros.
It's a normal bro thing when you're almost the same size after all! And if neither of them wears the other's stuff in public, it's just because they don't want to make it dirty.
It's just a bro thing when Bakugou give him back his red hoodie, the one that's a bit too large and gives him cute little paws (making it Kirishima's most prized possession) and says "it doesn't smell like you anymore". Kirishima does the same after all. It's manly to feel safe in your bro's scent and to find comfort in being surrounded by it. It's like a hug, and bros hug each others.
So really, the hardest part for Kirishima is to make sure Bakugou doesn't willingly give him the shirt, but it's easy to hide it among other stuff.
The problem is that the task was so easy that Kirishima lowers his guard. He hid the stolen shirt among his own stuff, so he doesn't give it back to Bakugou by accident, and ends up wearing it one morning for breakfast.
For his defense, it's a red shirt. He's only seen Bakugou wear it once, and it could have belonged to Kirishima in the first place.
He realises his mistake when he sees Bakugou's eyes go wide in surprise as he steps into the kitchen, but instead of being mad that Kirishima would risk staining his precious shirt, he smiles and looks all smug. And since he's a true bro, instead of outing Kirishima, he walks up to him and whispers in his ear "That's my shirt, Eijirou." It makes Kirishima shiver because it tickles, and it only gets worse when Bakugou kisses his neck before stepping back. They look at each other brovingly, and Bakugou says Kirishima can keep it just as Kirishima says a little too loud, "Sorry, I thought it was mine, haha!"
As if on cue, Kaminari and Sero rush into the kitchen, yelling and laughing, reminding Kirishima over and over that he owes them so much meat, and they only calm down when Bakugou threatens to explode them.
"Why aren't you mad at Kirishima though? He stole your stuff!" Kaminari asks him.
"So what? We're dating," Bakugou shrugs.
"We ARE?" Kirishima asks.
Oh... so it wasn't a bro thing after all, Kirishima realises. It suddenly makes Bakugou's bro confession a lot more... well, romantic.
"But we haven't kissed," he points out.
"You never fucking asked. I told you I'm ace."
"But I thought you said 'I'm an ace'!"
It takes a little while to clear the misunderstanding, and it would have probably been easier to hear each other if Kaminari and Sero weren't howling and giggling on the floor next to them, but it doesn't matter because Kirishima has a boyfriend now and it's great.
It's the only thing on his mind when they go to that BBQ and his friends suck his money dry.
