Your understanding does not match the scientific evidence. One of the reasons being trans is not a mental illness is that cis people experience gender dysphoria all the time as well, like cis men when they're called girls, cis women with facial hair, or the revulsion many cis people experience when they even so much as think about the idea of taking hormones that don't align with their gender. To the extent that this is a "mental illness", the vast majority of people have it.