Ty soon thought of what he'd like to try wishing for from this position first- particularly as it' probably be easy to do. Since he'd learned about how human reproduction works, something had intrigued him. The chance of someone being born a boy or a girl was basically even odds. So what if the figurative coin toss that led to his existence as Ty had gone the opposite way?
Ty focused on this idea- having been born a girl instead of a boy- and in an instant the sex that had preceded his conception played out all over again. A virile young man's raw cock pounded a young woman's pussy, their pleasure oscillating out to Ty's ovum form, and they peaked as the penis shot warm loads of semen, which flowed eagerly towards Ty's ovum form and began to chew away tirelessly at its walls.
At last, one pushed its way inside and fertilised Ty. And he could feel that it wasn't the one that had fertilized him the first time. It hadn't got a Y chromosome, but an X.
This sperm's genetic material being part of Ty made everything feel... very different. Ty could feel it imprinting itself on him, like a parasitic mushroom making a host's body do what it desires. Even though his wish allowed him to do whatever he liked, this felt irrepressible in the most arousing way possible. An unfamiliar command of sorts pervaded Ty's being: you are female now. And his body followed its guidance eagerly, and slowly became her body.
The image of the person Ty was in his mind's eye had started to change, metamorphosing from male to female. A firm jawline gave way to soft cheeks and puffy lips. His body shrank and became curvaceous. Callouses and firmness vanished from his skin and it became smooth and mostly devoid of body hair. Large soft breasts formed on his chest, and a desire to have them played with and sucked on lodged itself in his mind. His penis and testicles shrank away as they had no cause to exist anymore, and instead a vagina formed and connected to feminine organs inside. Even Ty's sexuality and gender were shifting- he became she, and she saw herself as a woman, she felt attracted to men and penises as her sexual opposite.
While physically Ty was still no more than a xygote, she embraced the girl she had become, and wished to spend some time as her new self. In response, her embryonic form developed rapidly into a foetus ready to be born- she noticed the change in her sex assignment when that part occurred- and she was born. Just minutes ago he had been a newborn boy, now she was a newborn girl. She then grew and progressed through a decade and a half, until she stood in her room, having grown into a teenage girl the same age Ty had been when he started his wish before as a teenage boy.
Ty looked down at her feminized self in the flesh for the first time, and was amazed at how everything both felt like normal and like being a totally different person. All that had changed about her was her sex, and yet she was so different from her boy self. She wondered what to do in her new reborn life...