why is it dehumanizing?
Because it sounds academic?
Sure, it sounds, a little awkward, but dehumanizing? Come on, even you are not that far gone to defend that ridiculousness. But it's not only academic, it's used in the news all the time. "The suspect is a white male" (ha, like that would ever happen). There's nothing improper and dehumanizing on its face.
It's actually because gross nerds use it in gross ways is the real reason.
See,
this makes you sound like a toddler, marrec.