From our beloved book (слэш)

He hadn't even really touched himself yet. Just once a few times. And it had all started when he found out that Izuku was sleeping with Bakugou and Kirishima.
It was an idea in his head that he didn't really understand and he had to research what threesome sex was like. And then he had to research again how threesome sex between men happened. He didn't want to see videos from strangers, which is why he oriented himself on descriptions and... it was disturbing, but at the same time very plausible.
Especially when he tried to imagine the whole thing with Izuku, Kirishima and Bakugou.
And with the idea of ​​them touching each other, kissing and caressing each other, his body just... Reacts.
 
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OR: Shoto was never someone who could handle or even understand emotions well. But when he was around Izuku, they seemed to make more and more sense and take shape. As the relationship between the green-haired boy, Bakugou, and Kirishima intensifies, he decides to step in. However, things don’t always go as planned.