Katsuki is a man, and even if he did not realize that at 5 years old like all the famous transgender people he saw on the internet, he is still a man. He hasn't told anyone in his class that he is trans and its not to keep a secret but more to keep the other's mindsets as they are. He doesn't want pitty or to be treated like a fragile flower in a group of men. So he keeps it a secret. But to Bakugou's misery, his true disires and pains shine through when someone else gets what he has always wished for.