Alex slowly realized what had always been off to him. His flat chest, his straight figure, his penis... he didn't think he liked his gender. He had never really allowed himself to seriously consider that possibility, but if he could use his power to make it so his transition would be seamless...
Okay. He'd try it. Alex took a deep breath and then said to himself six words he'd never felt he could before: "What if I'd been a girl?"
After he said it, a strange vision flashed through his head- a penis ejaculating inside a vagina, and sperm surrounding an ovum with one of them breaking into the egg and fertilizing it. But then time seemed to spool backwards, and before the sperm that fertilized the egg the first time could, a different sperm pushed inside.
Alex soon realized what the vision had been. He had just witnessed his conception- and the wish changing his conception. The first time a sperm with a Y chromosome had fertilized the egg, but now one with an X chromosome had instead. Once it took effect, he wouldn't have a male body- she would have a female body.
He shivered with anticipation as his body began to catch up with his brain. His undeveloped chest grew sensitive and his nipples swelled and got erect. He saw his figure reshape, turning shorter, more petite, curvier. Soon he had narrow shoulders and wide hips, and the wispy body hair he had had almost vanished. He winced and screwed his eyes shut, and saw his facial features had turned distinctly feminine, with soft cheeks and puffy lips.
As the changes reached Alex's genitalia, the sensations grew more intense. He breathed heavily as he saw breasts sprout on his chest, swelling mounds that he ached to have played with. And with a high-pitched whimper befitting his emerging sex, he saw the last masculine part of his body get subsumed by femininity. His penis shrank inwards, the nerve endings binding up tighter and tighter, and he felt his body castrate itself as his testicles were pulled up inside to form into ovaries.
While seeing his breasts continue to grow, his cock head recede into a clitoris and his scrotal sac reshape into vaginal folds, in Alex's mind the reversal of sex was unconsciously asserting itself. He would be she, not male but female, not a man but a woman. Also inverting was Alex's sexuality- as a man he'd felt attraction to girl's soft curves and breasts, but as a woman she would feel attraction to guys' firm figures and penises.
Slowly the changes finished, and Alex stared in amazement for a moment at her feminized body.