Josh wanted to get pregnant really bad, but obviously couldn't do it himself and knew it wasn't fair to force a girl to deal with that (not to mention he didn't have many girls as friends anyway). Now he had his power, he wasn't content to just wish to be a girl and pregnant- he had a much more fun way in mind. Josh's weirdest, most intense kink was parthenogenesis. The idea of someone being both sexes at once and able to impregnate themselves was amazing to him. It would be so cool to be self-sufficient like that.
Josh smiled and made a wish: "I wish that the next time I orgasm I would turn into a woman, and my semen would get me pregnant." He felt a shiver run through him and started to grow intensely aroused. He undressed and got on his PC to start looking up porn, and fapped eagerly, possibly the horniest he'd ever been. It didn't take long for him to ejaculate messily onto his hand, and he carefully aimed his load so it covered his hand, not a single drop sliding off.
Rather than feeling kinda tired like he usually did after his peak, Josh felt more intense feelings continue to run through him as his wish took full effect. Sensing that his penis had served its function, the shaft started to recede back inside him, his testicles following suit until he had a flat mound at his crotch. His chest developed similarly, his nipples enlarging and breasts forming as mounds of flesh around them. His figure seemed to grow cutely curved and his thighs widened. At last, between his- or rather now her- legs a slit formed from what had been his scrotum and had now developed into vaginal lips, and his penis shrank to its feminine size as her clitoris.
Josh looked at his semen on her hand, and smiled as she moved it down to her new vagina. She smeared some of it over the opening and allowed more of it to settle on two of her fingers, slowly pushing them inside of her and gasping and panting as she felt feminine penetration for the first time, loving the sensation.
Deep inside her body, his sperm had eagerly capitalised on her insertion and were racing up to her womb. Soon they came upon her ovum and energetically chewed away at its walls, desperate to fertilize this egg out of instinct, not knowing or caring it had come from the same person.
It soon capitulated and a sperm cell moved inside, fertilized it and it proceeded to lodge itself into Josh's womb, impregnating her. She had impregnated herself, and she loved how it felt.