I can't understand it. Your position is irrational and raises sex to some sort of mystical or magical experience. The guy didn't know the girl was trans. Obviously. He had no idea until somebody phoned to tell him. And then he still didn't believe it. He got a blowjob and livestreamed it, so he was obviously stimulated/not disgusted "enough" to maintain his arousal and pump into her mouth.

It's bad enough he was creating and distributing porn featuring a minor girl. But for him to be homicidally enraged because she was trans? Contrary to your assertion, that makes zero sense ... unless hating trans people makes sense to you. I assume hating trans people DOES make sense to you given your use of "biological male" a term limited pretty exclusively to transphobic bigots.