That's a little too close to saying that a person chooses their gender, in a quick read, when you mean that they place a higher priority on gender than sex. It is common for a person to decide that their physical body does not accurately reflect the real person who is on the inside. Most of the time it isn't so severe, even when surgery is involved, but the precedent is certainly in place. In many ways ideas are more real and vibrant than actual living flesh.