Gender is not the same as sex. "Passing" is just another way of saying "being accepted as." There are, for instance, a small number of post-op women who live as men. Their sex is female, but they "pass" for male. And many FTMs live as men without having bottom surgery, because it is not as successful as MTF SRS.
Nonetheless, I can't deny that it's better to have congruent equipment for any number of reasons, both practical and emotional, and I believe it when post-ops say it's an epiphany. I'd love to wake up in the recovery room with a brand-new vagina; and the reason I want one is that I'm already a woman.
Lallie