Jeff always thought it was dumb how human sex characteristics worked. Half the species could only breed for a few days a month, had to carry their offspring for 9 months, and were treated as inferior. How dumb was that?
So he had an idea to change things: every human would have periods, but they wouldn't be like periods in the old world. Periods would be the time in which men, the default kind of human, would become womb-men, and if they were fertilized in the 14-day period they were in that form they would birth their offspring in 9 hours. Plus, to even up the inequalities of sex, men would have mammaries capable of feeding their young, though they still became more pronounced when on their period, and all sex-related processes like periods and birthing would be painless.
Jeff made his wish, with the caveat that he wanted to be on his period so he could experience turning into a womb-man. Eagerly, he stripped off and stood in front of his mirror to get a good look at the change. Weirdly, the first thing to change was his mental state- it prepared him for the changes, asserting to him that this was natural, helping him adapt to the new feelings racing through him.
His balls were sucked up inside his body, and his crotch flattened around the point where they had been. His hips widened slightly and his butt enlarged. He noticed his face had subtly changed, his jawline softer and lips puffier, and his muscles slimmed to become more shapely. Unfamiliar instincts were brewing within him as he felt his chest enlarge and nipples swell, wanting to have them sucked and pinched.
Pretty soon, the most intense changes started in his crotch. He watched as his penis slowly shrank to a little nub, its nerve endings concentrating in a far tinier space. Behind it, a slit opened out, and he ached to take things inside it- no, not he, he realized, she. For she was a womb-man now, and would remain so until her cycle ended in a few days.