Ty had always felt like it was dumb how humans were divided by gender. The way men were treated as default and women as other by society, and how you could either get other people pregnant or get pregnant yourself depending on random chance... he felt like it'd be way better if everyone was the same. And so he wished, "I wish all humans were one gender."
As his wish spread throughout the human race, our species' biology was irrevocably changed. All humans developed wide shoulders and firm muscles for greater body strength, and wider hips for birthing young. The undeveloped chests of former men expanded out to form breasts that young can suckle on. The undeveloped clitoral nubs of former women developed the capacity to expand several inches and ejaculate, with testes forming inside the body; the penises of former men shrank to form phallic tissue that was identical, and their testicles moved into place while adapting to body temperature, leaving their scrotums to part into vaginal slits.
Naturally Ty had also transformed due to this, and hastily undressed to get a look at his (her? their? The pronouns of this world weren't clear, thinking about it) body. Ty felt beautiful like this, admiring his new tits, his raging erection and how his pussy was dripping between his legs. He wondered what to try first in this transformed world...