My point though is that if a person renders themselves unable to have offspring, especially before they have any, then from a purely biological perspective it's a form of illness. The same view would apply to all animals. If every member of a group acted the same way then the group would also die out. The functional impairment you mention includes the inability to attract a mate and reproduce. Hormones and sex reassignment surgery also severely hamper fertility.
Also trans people tend to have many other correlated mental issues and for example have high suicide attempt rates.