"OK, first you need to find a partner," the teacher says. You look around and see boyfriends and girlfriends are generally partnering up, and you go up to Mina, a girl you don't know too well, and ask her.
"Alright," she says casually, and the two of you sit on a mat together, waiting for everyone to get partnered up.
Once everyone has, the teacher gives the next instruction. "Now, you all need to undress. Get comfortable seeing each other's bodies, because understanding them is a crucial part of healthy sex."
To your surprise, Mina pulls off her top pretty quickly, while you fumble with yours, and she smiles as she sees you trying to hide your hard-on. "You don't have to hide that, you know," she says, kindly but a bit insistently. Nodding, you pull your shorts off, and she looks enamored at your hard cock as she removes her skirt and panties, standing naked in front of you.
"Right," the teacher says once everyone's undressed. "So, to start things off with a bit of fun, let's do an exercise called 'The Old Switcheroo'. He holds up a remote, points it at all of you and presses a button on it.
You instantly feel kinda strange. Looking at Mina, you notice she seems confused too, and you both start breathing heavily, realizing your bodies are filling with weird hormones. "Obviously, at your age you all kinda know how your own bodies work," the teacher says. But it's unlikely you're as familiar with the opposite sex, so using this remote, I've switched all your sexes! It takes a little while to take effect, so pay attention to the changes. Boys, figure out how you like having your chest touched as your breasts start forming. Girls, take a note of how having a penis feels compared to a clit."
He's giving more advice, but all you can really focus on is how you feel, how your body is feminizing and how Mina's body is turning masculine. You moan a little as your nipples enlarge and your chest swells, and for some reason seeing Mina develop a firm, flat chest is really arousing to you. You feel your balls move up inside you and your crotch smooth out as Mina grunts and pushes two lumps into her newly-sealed pussy, and as your shaft shrinks and recedes into your body you watch Mina's clit swell and expand until it's more than big enough to fuck someone with. At almost the same time, your body opens out into a slit and her cock reaches full mast.
You quickly realize that remote has turned your sexual attraction on its head as well as your body. Before you thought of yourself as totally attracted to women, but now you feel an irresistible need to fuck the man in front of you, the man who used to be Mina. You smirk teasingly at him, lie back and spread your legs, and a split second later he's atop you and you feel his cock head rub your clit.
He moves it downwards, and as it slides inside your warm wetness slowly you feel an intense, spreading warmth as he fills you. The firmness of his hands and body holding you down excites you. So do his eager grunts and pants as he picks up speed and starts to rail you. When he reaches to play with your newly-formed breasts and cups them and pinches your nipples, it feels even better.
Everything about this feels so impossible, and that's why it feels so indescribably good. The man fucking you was a girl, and was made virile, given a cock with which to fuck you. You were a boy, and were made nubile, emasculated and castrated in exchange for the intense pleasure of womanhood. In your head the memory of your breasts growing and your cock receding and his cock emerging repeat rapidly, as does your visualizing what him pounding you looks like to an outsider...
Mina grunts, and you cling to him as he cums inside you. You feel yourself cum hard from his ejaculation, and the two of you lie there coming down from the high. Fuck, he makes a good man.