"You want to make a convincing girl? I can help with that," a voice says in your head.
You're taken by surprise, and look around confusedly. "Who said that?" you ask.
"I did!" the voice says. "I'm a servant of Inari. It's a deity of many traits, including fertility, and has multiple sexes. You know what that means?"
"No?" you admit.
"It means as a servant of it I can use its power to change another's sex- I can make males female, females male, or turn a body androgynous!" It hesitates for a moment. "Would you like me to turn your body from a boy to a girl?" it asks after a moment.
You're still kind of shocked by what it's told you, and it existing at all, but you know for a fact you want that. "Yes, please," you murmur.
A moment later, you see your body in the mirror, suddenly naked. You jump, and look down to see you're still in the yukata. "Sorry, I just wanted to give you a view of your body as it changes!" the spirit explains.
"It's fi-iiine..." you reply, your voice cracking as the changes start. Pleasurable shivers run through you as you feel yourself seem to reshape in places. Your skin seems to soften and smooth all over, and your shoulders shift inwards slightly as your thighs spread out.
"Good girl," the voice tells you softly. "Embrace your femininity. Now you see how malleable human sex is, that changing one part of one pair of the chromosomes causes the body to transform so." You're surprised to realize that hearing its voice goad you on doesn't feel uncomfortable, but affirming.
"Would you like breasts?" the voice purrs. A high-pitched whimper falls from your lips and you nod, and feel your chest grow sensitive. "Doesn't this part feel nice? To have an erogenous zone where once your body was barren? To have something on which your offspring may suckle?" You pant in agreement as you sprout breasts, your nipples growing more and more sensitive until B-cups have formed upon your chest.
At last a tingling sensation surrounds your crotch. "Now for the last part. Relinquish your penis and testicles," the voice instructs you. You can't refuse, you don't even want to refuse, and you feel yourself become emasculated. Your balls move up inside you and rapidly reshape into ovaries, your shaft shrinks back into you, and your scrotum tucks inwards until an opening forms at its centre and connects to your new vagina. In the mirror you now see a naked girl, one who looks much like you except she's clearly not a boy.
"You look beautiful," the spirit's voice says, and you look down at your newly feminized body and start to walk outside.