"You know, there's something else I can do to make you more comfortable."
You perk up. "Just...wait here."
She steps back and starts undressing, until she's completely naked before you. As a girl, it was perfectly appropriate for you to see her like this.
She changes some settings on the machine, but before the process is launched, she pulls you a bit closer to talk. "I need you to trust me, because I'm going to do something crazy after this. She then pushes you back as you hear the whirring of the machine.
The changes begin quickly. Your girlfriend's skin is now both a lighter and darker shade that matches the color of your own. Her hair turns ebony black and her boobs are now A-cups, the same size as yours. Judging from her height, she's now five years older than you.
Stepping out, the first thing you notice is that your girlfriend is having trouble making things out at a distance. "Shit, I knew I forgot something. Good thing I made sure to pack these." She takes out a pair of glasses from a case and puts them on. "There. From this moment on, I am your older sister." You picked up on the fact that she was now speaking Japanese fluently. And you understood every word.
"I programmed it so that we're both bilingual. Now we have our own little secret language for when we want to communicate privately. Come, we have some things to do."
A hour or so later, both are you are facing each other, naked once again. Your hair is now made up in one long braid against your back and hers are in two side ponytails.
"This is going to feel weird, but I need you to trust me." Suddenly, your body starts to stretch upwards, your boobs melt away, and you feel your body bulking up. Then you see it: a big, fat penis hanging limp between your legs and coarse hair on your crotch. And your sister also grows, until she's a full-grown woman with ponytails. And you're a man with a long braid going down his back.
Smiling, your sister grabs your penis. "Sister, we need to milk your semen. I'm going to teach you how to be a man, so when I change you into my husband, you can act like one. But for now...just relax."
As she kissed you, you felt drowsy and your eyes slowly slid shut. When you opened them, you were a little girl again. And your sister was your sister once more, only now she was holding a cup filled with thick white semen.
"Now it's time to play doctor, little sister. Fertility doctor."