White Christian cisgender heterosexual men believe that they represent the highest level of evolution. You'll notice that any deviation from that configuration is seen as "less than" at the very least and "satanic" at the worst.
Having your little "boy" decide to "downgrade" from male to female is more than they can bear. "How could little Tommy do this to us?"
"Who or what did this to little Bryce?"
"It must have been the school"
"...video games"
"...the internet"
"...democrats"
"...TV and movies"
"Now we must disown him/her because they've hurt our family"
ANY answer other than the fact that their brain isn't wired like you thought it was is fair play.