I don't know who said you could lose all sexual function
since erections are not directly related to T, but to arousal.
T, just makes it easier to get aroused and
increases the desire for sex, so through the
brain's action, the penis gets erect.
The main sex organ is the brain and most sexual disfunction
are psychological rather than functional.
T levels has much more impact on the reproductive capacity
of the testes. But, even there, it mostly impacts
sperm count (meaning, there's always a very slim chance
of getting someone pregnant even after years of HRT).
Many MTF hate their penis a lot, and thus cannot really get aroused,
so once T is removed, they're left with no libido. High E levels
can bring back this desire for sex, but often MTF are underprescribed.
Myself, I can easily get an orgasm in less than 10 minutes if
I use a bag of well worn fantasies. I can do it several times
in a row (10 min in betwen) even (I could do the same prior to HRT).