If you read the manga correctly. He as an individual didnt use the term in a negative way. But the people AROUND him did, they literally laughed and shit talked him by calling him such names which is why he took it as an insult. He never actually said hey you being a slut.whore is unacceptable. He said you letting them laugh and taunt you by using your truth is unacceptable. It’s a small but important detail to note.

idrk how to word this but he’s a whore/slut yet u slept with him. very weird how u use words with negative connotations to demean the uke but u wanted to be in his pants. ur brother assaulted him in his sleep, he’s not innocent. this protective act i kinda get because of how the seme grew up and such but this is excessive.