We may not necessarily expect the average person to fully understand transgender existence even if they aren't against it, but Doctor Ben Carson is not an average person.
Carson spoke to reporters at the GOP convention where he referred to transgender people as "the height of absurdity."
"For someone to wake up and think that they belong to a different sex because they feel different that day is the same as if you woke up and said I’m Afghani today because I saw a movie about that last night and even though my genetics might not indicate that, that’s the way I feel, and if you say that I’m not, then you’re a racist,” Carson told [The Hill].
That's not actually how it works. People don't just wake up from one day to the next and decide what gender they are based on how they feel in the morning. The real "absurdity" here is comparing gender identity to skin color or nationality. "Afghani" is not a race.
Unfortunately, Carson is far from alone in the medical field in having little to no understanding of transgender identity. But Carson is not your average doctor, he's a goddamn brain surgeon. He should understand this better than most, but he doesn't.
Carson's ignorance probably shouldn't come as a surprise given that he also believes gay people are capable of changing their sexual orientation on a whim.