As he explained he neatly wrote down everything on the whiteboard perched up onto his wall which they sat in front of.
"Voluntary, and Involuntary, a lot of times  people who involuntarily regress automatically presume that it's a trauma response and this is how their brain is coping, but-"
He scribbled down a short summary of what he was explaining, making sure to define what Voluntary, and Involuntary meant, so kirishima could keep track and not have a single excuse to be confused. It's all laid down in front of him, he shouldn't get confused.
"-It could quite possibly be mistaken as a symptom for something bigger than this" he stopped writing, making sure kirishima was still listening.
"Aaaaaannd....how exactly does that relate to your question?" Kirishima asked hesitantly, katsuki sighed "the problem is voluntarily choosing it to cope" he grit his teeth then grabbed the laptop from kirishima's grip, scrolling up to the slide where he placed the screenshots of his...experiment.
"Look here" he turned it around so he could read it properly "ignore my cringy ass texting but you see how this guy reacted to it?" Kirishima wanted to look away, this was gross. And not in the sense of 'ew age regression is such a weird and kinky thing to do!'  
As in this dude is a disgusting and revolting human being and should be burned alive. 
He looked at katsuki in need of an explanation, he rolled his eyes "calm down he didn't do anything that bad, well at least not to me..." kirishima frowned.
"Let me start from the beginning..."
He cleared the white board, tossing the eraser on the ground before opening a marker.
"Remember that girl that was interested in my researches a while back?" Kirishima thought as katsuki drew a circle with what looks to be a chicken in the middle "yeah the one you kept complaining about? The thirteen year old girl?" He nodded.
"She was interested in a research that I've done a while back, and it reminded me of the age regression topics I put away" he frowned, he put it away for multiple reasons, one of them being he felt conflicted. People clearly expressed their distaste for this subject and yet people still try to show the positives of it...so why?
"One day I brought up the question again but I wanted to hear her opinion, form a very excepting perspective. But It backfired and led me to...that" his eyes glared at the screen, where the screenshot lay open for everyone to see.
Thankfully it was only kirishima witnessing this.
He gulped "go on" fully open to the situation kirishima listened. " I got so-so angry! Why is this brat calling me stupid!?! Who the hell does she think she is!?" He huffed "my decision wasn't well thought of ,I know , but I got the outcome I wanted I don't care that I had to deal with that revolting piece of shit of a human being just to get it!" 
He added two more circles onto the board.
"I got in to an age regression gc, because I came to a conclusion that in order to prove to this ass that I was right and that I wasn't some dumb idiot!! I had to have full proof, and that came in the form of being in the position of a babysitter, a caregiver and a little" kirishima's eyes quickly went back to the screen, is he being serious!?
He put himself through THAT! To prove a point?!
Damn.
"The babysitter thing was easy, just talk it's one littles in the gc, the caregiver thing was a huge ick knowing that this person was actually searching for comfort and I'm just studying them like they're my Guinea pig or some shit-" 
He rolled his eyes continuing "but then came the last part, the part where it would really prove my point that Age regression has way too many flaws and if you voluntarily choose it you either got some underlying fetish! Or you didn't think it through and now you have to deal with the consequences" he dryly laughed, gently tugging at his hair and looking back at the silent redhead.
"So...how'd that happen?" The blond sighed "I got two caregivers, one that I was skeptical of and knew he had some toxicity to him, and the other who was a genuinely sweet guy who just wanted to help and be friends" he sighed clearly disappointed and seeing how this'll all work out.
Katsuki kept scribbling on the white board "of course the toxic guy was shitty but he hid it well, when I claim that he's shitty I mean he's a pedophile who's only interested in grooming" he stopped writing at that, holding the marker real tight.
His eyes showed a bit of hesitance but it quickly faded away "he asked if I was uncomfortable with anything in particular, but I wanted to see how far he'd go so I said I no, I didn't consider that I might actually get uncomfortable...
I mean I've handled a ton of shit so why would this conversation through text affect me?"
He looked at Kirishima with a desperate look, he stopped writing again turning his attention fully on Kirishima "he admitted things to me, things that I wish I could track him down and arrest him myself... he was a villain for sure but not what we're used to" the texts in front of them were enough proof that katsuki didn't need to elaborate on it.
"He had some odd interests ,realised he loved feminizing..." he didn't want to say himself, he didn't want to put himself in the spot, to relive that feeling.
"It's shitty" he sat closer to kirishima and fiddled with the marker in his hand "we role played of course ig that's how things always work over texts now a days" he scoffs "he kept on praising me and saying he loved me and of course he'd cuddle me, touch me, kiss me..." he chuckled "how can a person make you so uncomfortable through texts?" He questioned looking up at kirishima for answers.
This is getting to long so their will be a part two.