Katsuki had been aware of Kirishima's affections early on, almost from the start. One could have little interest in romance but still recognise the signs of attraction.
At first Katsuki had ignored it. Fucking your friends never ended well. It would be a bad move, Katsuki reasoned, even without my physical…inadequacies. As if he would wreck one of his most authentic relationships just because he appreciated the hard lines of Kirishima's body. It was easy to tamp down the desire he felt towards his friend when he thought about what he stood to lose.
He cannot pinpoint the exact moment began reciprocating - does not know how it happened, when it happened, only that it did. Katsuki thinks part of the reason it took him so long to identify his romantic feelings was because falling in love with Kirishima was so unlike the depictions in those cheesy movies Racoon Eyes loved or the experiences of his friends. It was not like finding a missing piece of himself or finding his other half. It wasn't the gradual shift from friends to lovers or the sudden realisation that 'yes, I am in love.'
In contrast, actually being in love, conformed to every cliché in the book. Katsuki lost himself in Kirishima's beautiful expressive crimson eyes, lived for his laugh and treasured every touch. He craved those private moments between them when he felt so, so connected to the object of his affections.
It took all Katsuki's self-control not to break down and confess. Although most people feared rejection after confessing, Katsuki feared his rejection would come later when Kirishima saw he naked for the first time. Starting a relationship with an expiration date was stupid and reckless and the heartbreak from a break-up would hurt even worse than a rejection ever could. The uncertainty if even their friendship would survive led Katsuki to devise a system that would prevent himself from destroying everything. Whenever he began to have delusions about confessing or thoughts that even a brief relationship with Kirishima would be worth it, he returned to the back alleys of his middle school years. Nothing quite stamped out his self-confidence like having somebody grind their shoe into his crotch and verbally degrade him as they roughly fucked his face.
Sometimes he thinks – thinks Kirishima wouldn't care. That he would love him anyway. Then like a bucket of cold water thrown over him he recalls all the evidence he has to the contrary.
Like when Bird Brain had come out as a sex-repulsed asexual to the entire class. The class had bombarded him with questions. Surprisingly, he had answered all his classmates' questions, even the pointless ones that he would usually directly ignore. Katsuki had learnt a lot that day, in fact Katsuki had been so inspired that he had resolved to stop calling Tokoyami 'Bird Brain' from then on. That night Katsuki had diligently researched asexuality and had restructured all his preconceived ideas about sex and relationships in general. Crawling into bed, Katsuki wondered why he ever thought that sex was the basis for a successful romantic relationship.
He remembered why later that week.
Several days after his epiphany Katsuki overheard Kirishima congratulating Tokoyomi on his new relationship with general studies student from the year below.
"I'm so happy for you! I still feel really bad about before…"
"Thank you. And no need. You taught me an important lesson – that one must compensate for difficulty by gathering information. With the added context I have now, I see why a relationship between us would be ill-advised."
"I mean you are a great person! And your identity is valid and – "
"Kirishima, I assure you that your rejection is seared into my mind in the utmost clarity. There really isn't any need for you to repeat it" Tokoyami said with a smirk.
A look of horror crosses Kirishima's face until he registers the sparkle in Tokoyami's eye. "Sorry dude! Urgh, I didn't even mean to make this about me!" Smiling sheepishly Kirishima rubs the back of his neck. "She really makes you very happy doesn't she? I'm glad you found your happy ending. I knew would!"
"Indeed. Even though we only just started dating, I can't envision my future with anyone other than Akiko being by my side."
"Damn, on a first name basis already?" Kirishima punches Tokoyami's shoulder affectionately. "I really am happy for you man."
The two boys are quiet for several moments, apparently conveying some private sentiment through their eyes until Kirishima hesitantly holds open his arms.
"So we gonna hug it out or what?"
To Katsuki's great shock Tokoyami steps forward and warmly embraces the spikey haired idiot. It is a tender moment but Katsuki feels nauseous. He abruptly turns away and heads back in the opposite direction, crashing into Deku when he rounds the next corner. Deku dramatically falls on his ass with a pitiful squeak but Katsuki cannot find the energy to engage and keeps walking. It takes him awhile to shake Deku off but once he has, he walks outside and sits with his back to a tree. Staring at the dirt in front of him, he reflects on the conversation he has just heard. Katsuki knows for certain that:
1) Tokoyami confessed to Shitty Hair.
2) Shitty Hair rejected him because he thought they were incompatible.
Neither of them said it directly, but from what Katsuki can infer from their conversation the 'added context' Tokoyami referred to suggests Kirishima believed the two to be incompatible because he was unwilling to be in a relationship with somebody who would not sleep with him. Honestly Katsuki does not know what surprises him more; that Tokoyami feels close enough to Kirishima to confess, that Kirishima didn't tell him about said confession or that Kirishima is apparently such a sex fiend that a relationship without it is a complete deal-breaker for him.
At one point, Katsuki had thought, thought that maybe a sexual relationship between them could still work. Unlike Tokoyami, he was not sex repulsed. Yes, Kirishima would undoubtedly be disappointed but there were other ways to get off weren't there? He could suck dick like nobody's business and if Kirishima was that fixated on getting penetrated, well there were toys for that weren't there?
This time the incident that dashed Katsuki's hopefulness occurred during a weekend outing to the shopping district. This trip had incited Katsuki's irrational hatred towards Hagakure, as she was the one who suggested the outing. At first Katsuki had enjoyed walking around with Kirishima, even though Denki had insisted on tagging along. Towards the end of their outing Kirishima had disappeared claiming he needed to buy underwear and would prefer to buy it alone. Katsuki had rolled his eyes but had agreed to wait for him in the food court with Denki. When Kirishima returned 20 minutes later, looking flushed and clutching a black bag Katsuki hadn't thought much of it. It was only on the journey home when Mineta made a comment about seeing him in the red-light district, that Kirishima came clean. He shamelessly told them he had gone to buy a dildo, because his hand wasn't enough and he'd have to make do until he could find a boyfriend or regular hook-up. The boys had all crowed and demanded to see what Kirishima had bought.
"C'mon now Kirishima! Now's not the time to be getting shy" Sero chortled.
With a good-natured sigh, Kirishima had glanced around the train carriage, confirming, that it was indeed just them. He then reached into his bag and when he pulled out his acquisition, Katsuki's stomach dropped.
Even the others were shocked into silence.
Kaminari was the first to recover, "Dude! What even is that?"
"It's called the Blunger! It is made by a famous American brand but I found a legitimate retailer here in the city! Took some pretty intensive internet searching though, haha."
Mineta's eyes practically bulged out of his head, "It's huge. I never thought you'd be such a size queen Kirishima"
"I mean I wouldn't call myself that…honestly, and forgive my crudeness, I always prefer to be fucked by a human dick and I certainly don't expect anybody to be packing anything sized like this! But when I have to settle for an inanimate object I like it to be bigger to compensate. This has way more give than a human appendage though, look!" Kirishima gives his dildo a demonstrative squeeze before dropping it back into the plain black bag.
"But still! You must like them…big, to be able to appreciate that"
"Oh yeah for sure! I need somebody above average to satisfy me but - "
Katsuki didn't catch the rest of the conversation, opting to glare out the window instead. He had never felt so worthless in his life.
In his younger years Katsuki's embarrassment primarily stemmed from his belief that others would see him as inferior. It was only later when Katsuki became interested in sex and boys that he had begun to worry about his ability to satisfy his partner. But now? When the person he was hopelessly in love with was unknowingly telling him that toys and blow jobs were nice and all but for him the pinnacle of pleasure was apparently being penetrated by a big dick. The one thing Katsuki could never even hope to provide? Katsuki feels nothing besides absolute devastation.
Katsuki still vividly remembers how he watched the cityscape rush past the train window.
How he tried his best to tune them out, tried his best not to cry.