questions of sexuality, like if you're a woman (or think you're a woman), sticking your peen in someone is definitely the sex act that men do so it probably induces dysphoria
that and once you've been on hormones for long enough that shit stops working properly, note the shrunken genitals
Adrianna Nicole, Mandy Mitchell
Kitty Kaiti, Christian
Mostly the first reason though. If you're trans and for whatever god forsaken reason love your dick, it's completely possible to not let the hormones affect your ability to get hard.
>inb4 folder of literal trap photos