Young adult Izuku who is panicking over and questioning his feelings for Katsuki, and thus, it might mean that he's not as straight as he thought he was. So he wants to analyze, to study and understand, like he always does, which means, research. Looking for sources. Seeing how others have come to terms with their own sexuality, and how they discovered it.
But getting frazzled because the two explicitly gay people(specifically, cis men, in this scenario) he knows of are Katsuki and Aoyama...
In his mind, he can't possibly ask kacchan bc he can't divulge his feelings yet! It would be awkward since Kacchan is the only man he is aware he might feel a certain way about! and he really can't ask Aoyama simply because Aoyama pretty much walked out of the womb knowing he was gay so it is a different experience than growing up thinking you're one thing then finding out you might not be.
Additional thought! He talks to Aoyama anyway, and one thing leads to another, and Izuku tags along to a Pride event with Aoyama and Tooru(In addition to just going for fun and camaraderie, they do charity/community work through their agency and go every year!) and maybe even a few other 1-A friends, like Ochako, Tsu, etc. And Izuku is able to get a sense of the community!
He loves it! It's a good time! And through some conversations he has with friends and with strangers, he's approaching the understanding that he may be bisexual, actually?? And Ochako is like "Why didn't you tell me about all this sooner??" Because she is covered in bi flag colors herself and wearing her hair up in a messy bun suspiciously similar to Himiko's buns.
Ochako gets a bi flag for him at some point during the day and slings it around him, so he's wearing it like a cape, and Izuku feels...good about it! He's all smiles! He's making discoveries about himself, finding support, and having a wonderful first Pride. The one thing that would make it better would be...
Thats when he sees Great Explosion Murder God DynaMight on a small stage doing crowd work. Signing autographs, high fiving little kids, taking photographs with fans where he bares his teeth, scowls and flips off the camera- at the fans' request(they know what he's about), and at some point providing some small fireworks shows for the spectacle. He's decorated with rainbows too, his gauntlets have a new, green/teal/blue finish on them of the male gay flag for the event.
Turns out that Katsuki has started working the event for charity as well, and being Best Jeanists sidekick at the time, also reps the agency!
Izuku watches him work for a while, the fans eating up his moody demeanor as his trademark, finding such appreciation for DynaMight's work until he's basically just staring with heart-eyes... and then Katsuki suddenly spots him.
He suddenly acts moody that no one(Specifically Izuku) told him that Izuku was coming to the Pride event, and more so, WHAT THE HELL, DEKU, YOU'RE BI AND DIDN'T TELL ME!?
He takes it slightly personally, not because of his own latent, and hidden feelings for Izuku, but because he wants to be someone Izuku trusts, and it kind of wounds him to think that Izuku might not have wanted to come out to him, his childhood best friend and also someone in his life who is out as a gay man.
Ensue the miscommunication followed by explanations ("I wasn't hiding anything, Kacchan, I just didn't know until today!") and then understanding, maybe even a confession, if Izuku is feeling up to it, and if Katsuki presses about what brought him there.
They spend the rest of the event together after Katsuki is done with his part on the stage, Aoyama and Tooru (and Ochako + Tsu) leaving Izuku to his newfound discovery.
People do see them and photograph them together, and rumors start flying online.
Katsuki does nothing to argue with the rumors. Izuku is a little more busy explaining to his mom after she'd seen said rumors and photos, about 1. his new found realization, 2. Why he hadn't told her yet(much like Katsuki's reaction) because she loves him no matter what and just wants him to feel safe and happy, and 3. explaining to her that no, Katsuki and him are not dating(yet), it's a little more complicated than that at the moment.
Katsuki wastes no time, and starts romancing him in earnest the very next day, stopping by Izuku's classroom at lunch, determined to change that "it's complicated" to "dating".
(They're dating and Izuku is considered "out" within a week.)
