It could be that he has different values or aesthetic preferences that clash with the themes and styles commonly found in BL manga. Or maybe he's influenced by societal biases and stereotypes.
Maybe he has some deep-rooted prejudices or just doesn't understand the genre.
Perhaps he has had bad experiences related to it before, or he simply isn't open-minded enough to embrace the diversity and creativity in the world of BL manga. Also, cultural and personal background can play a role in his defiance.