"Gender swap," you read. "Wait, that- that's impossible, though!"
"Hopefully not now," she smiles. "I've tested this formula on myself, but I want to see how it affects a male body. It should be totally harmless and work just like mine."
"Just like yours?" you repeat confusedly as a strange sensation spreads through your body.
"Yep," she answers, and you blush as she casually unzips and steps out of her dress and starts to unbutton your shirt and unzip your pants. "Enjoy the show."
A few seconds later, you start to notice the changes. The pitch of Miss Shaw's voice deepens, and your chest feels really weird. You notice her larynx bulge, and look down to see your nipples have turned hard and swelled up. You notice her boobs seem to have started to shrink, and as you look down you see yourself start to grow them.
Her body seems to firm up and thicken while yours slims and turns curvaceous, and as hair sprouts all over it you're surprised to find you feel like she looks hotter with it. You let out a high-pitched groan as your figure seems to slim and soften as if in response to hers.
In almost perfect sync, your crotches both start to transform. She grunts and seems to be pushing something from her body, while you feel an empty sucking trying to fill you up. Her vagina seals up and two lumps drop inside it, stretching the skin where it had been, and you gasp in a higher-pitched tone as your testicles hunch up and push inside your body, leaving your scrotum to sink inwards with their absence.
Finally your penis shrinks away into the body, while her clitoris stiffens, swells and grows several inches. Soon your bodies have finished inverting, as your cock becomes your clit and a vaginal slit opens at the centre of your scrotum, while her clit finishes expanding into HIS 7-inch phallus.
Your eyes can't look away from Mr Shaw, transfixed by his handsome man's body, and he has to wave his hand in front of your face to snap you out of it. "How was that?" he asked. "Do you like your new form?"
"Yeah, it's really nice," you say. "Yours is, too."
"Thank you," he replies blushing. Entirely by instinct, you both lean into each other and kiss. Even though you both know full well this is wrong, the rational part of you is drowned out by the unfamiliar hormones which populate both your bodies, and your female juices are flowing at the thought of having his penis inside you.
Carefully, he brings you both to the floor, and positions himself against your slit. His newformed phallus slides into your fresh vagina as naturally as if you'd been born with them, and he covers your mouth as you moan from feeling penetration for the first time.
Pretty soon, he starts thrusting in and out of you eagerly, and you're both transfixed by the stark contrast your new sexes give. He's amazed by the irresistible firmness and urge to fuck that his penis gives, and you feel spellbound by the supple sensitiveness of a girl's body, things like his eager thrusting and your clenching around him, his cock twitching and leaking precum and your nipples and clitoris being erect and tensing feeling outside both your control.
Finally you both hit your peaks and your bodies fulfill their deepest instincts. His cock pulses and shoots semen deep inside you as he cums, and the touch of his warm seed inside you and of his firm phallus and balls spasming makes you hit your own peak. For a moment you feel like you never want to go back to being a boy- not if you can feel so good!