"I think I wanna start by fucking you as a guy. That way, you can find out what it feels like for a girl and I can find out what it's like for a guy."
"Sounds fun," you reply, and before you know it, she's turning back into a man. Her slim muscles firm and thicken, and her hair shortens while her jawline sharpens and her larynx swells, deepening her voice. The breasts on her chest shrink until she has pecs flush with her stomach, a straighter masculine figure having formed to replace her curves. Her shoulders have widened and her hips narrowed, and lastly she grunts and you watch her clit protrude and thicken several inches while her pussy seals up and forms a bulge which two lumps move inside.
You look over his handsome body, and then glance in the mirror at your own. It quickly sinks in how you've both transformed, and your feminized body reacts instinctively. Your nipples and clitoris become hard and your already moist pussy is practically leaking onto your inner thighs. You turn to face him, neither of you looking at each other's faces but instead at your bodies.
Your most primal sexual instincts have been reversed, and your unfamiliarity with the opposite sex makes them feel irresistibly strong. He needs to impregnate you, and you need to be impregnated by him.
Once you're touching each other, you kiss passionately and he pushes you to the bed and gets atop you. You spread your legs for him, aching to have his cock, his cum, his semen inside you. He pushes his shaft into your vagina, and you both groan in excitement, holding hands as you get ready for him to start moving inside you. He soon does, and it feels indescribably good.
To your surprise, it isn't just the physical feeling of experiencing a man fucking you in a woman's body that feels good. It's also the sight of this man who was your girlfriend, his gorgeous changed body moving above you, his concentrated face and grunts and pants of pleasure. And it's also the knowledge of what this room has done to you- how it's turned your girlfriend into this irresistible man, and turned you into this beautiful woman, and soon he's gonna fill you with his seed and impregnate you.
It isn't long until feeling and thinking all this brings you to your peak, and he only hastens it by cupping your breasts and pinching your nipples. Still not totally used to your vagina, you clench around him forcibly, and he groans and you feel his cock twitch as he shoots his seed into you.
You look up at each other, breathing heavily, and kiss messily as you hug each other closely and drift off to sleep, his cock and seed still in you.