"I have a suggestion for how to start if you like," your girlfriend says.
"What's that?" you ask.
"How about we start by both changing into the opposite sex at the same time?" she proposes.
"Do you think that'll work?" you point out. "This is the person ward."
"Well, if we're different people, it should," she says. "I guess we'll find out."
You both strip off, stand looking at each other naked, and focus on the idea of the change. Seconds later, you're both transforming into models. Her face becomes firm and handsome, and yours smoothens and turns prettier. Her upper body grows more muscular and her breasts recede into pecs, while yours slims and you feel breasts bud on your chest, his eyes drawn to them. Defined muscles emerge faintly in his torso while you develop soft curves, your skin looking clearer as much of your body hair vanishes. Between his legs, his pussy seals up and he pushes two lumps inside it, just as those same lumps move from your own sac inside you. A protrusion expands above his new sac, and as your own shrinks away, you feel a sucking feeling as a passage emerges leading into your body.
You look at each other's bodies in amazement as the changes finish- his eyes wander to your breasts, your own to his penis- and then kiss passionately. He picks you up in his firm, warm arms and lays you down on the bed, getting atop you and positioning his shaft carefully. Your eyes run over his handsome body- you've never felt as attracted to anyone as you do to him right now.
Once he pushes his shaft inside you, he kisses your head and starts to pound you eagerly. He grunts and pants in his deeper, masculine voice and you moan in your higher-pitched feminine one. Your brains are still processing the transformation and get easily- willingly, even- overwhelmed by the dramatically different feelings and instincts of your new sexes.
It isn't long before both of you hit your peaks- he grunts and cums hard inside you, and you cling to his throbbing phallus, your nipples and clitoris pulsing with sensation. After kissing sloppily, you fall asleep in each other's arms.