If you care about what I think, I think the transgender couple never got their sexual needs met as children.
What's the problem? Our development stages as children have different sexual needs like breast feeding and potty training. Maybe they couldn't satisfy their bisexual/transexual urges as a child, maybe they got confused with parental abuses added to it.
Whatever it is something went off course in their childhood to where the boy wanted to fulfill his unconscious desire to mutilate his penis, maybe in order not to be in a position of dominance, and the girl got some real penis envy by first seeing a boy naked. My own first encounter with a naked girl was when I was 6 where for some reason we undressed ourselves and were curious over our genital parts. All I saw was a slit and never understood its function. We didn't touch each other but all I can remember is that we stared at each other for a long time. She spent the night at my house, and it wasn't uncommon to spend the night at girls' houses at my age. I had a very normal development as a kid, nothing unusual or traumatizing. But Freud totally saw this as normal.
Really the "girl" who mutilated herself must be in such extreme denial over the genital fact that "she" has a penis. "She" is still very much a man with the presence of the penis, and no amount of surgery is going to remove that.
Intersex is more interesting to me, with the development of breasts, some genital ambiguity with the micropenis, etc.