"Babe?" you ask once you break the kiss.
"Yeah?" he replies in his deeper voice.
"It's so, so sweet that you'd do this, and, uhh..." You blush, and he smiles with a teasing expression on his face. "I kinda wanna try changing myself now."
"That sounds cool," he grins. "What do you wanna turn into?"
"Well, you can stop me if you really don't want me to do it," you say, stepping back, pulling off your clothes and preparing to wish. "Is it OK if I change myself to give you a 'bisexual experience'?"
"Sure," he says a bit confusedly, and as you start to change his eyes glaze over in arousal. You pant excitedly as hormones surge through you while you change- so this is why he said it felt amazing. You feel your body soften and shrink, your figure turning curvaceous, your hips widening, shoulders narrowing and your muscles turning more lithe and thinner.
You feel a shiver through your face as your facial features adjusted, and your hair lengthens to partway down your back. But the weird feeling from this was nothing to what you feel in your chest- your nipples perk up and grow sensitive, and as you breathe heavily, soft breasts slowly form around them. You groan in a much higher-pitched tone than before as you feel your balls move up inside you, your cock rapidly tugging back into you and growing more and more sensitive as it shrinks. Soon, it has receded into a clitoris, and you feel your scrotum become flush with your crotch and then form an opening, where it connects up with your former testicles (which have rearranged into your ovaries) and your womb to give you a vagina.
"So, am I hot?" you grin once you recover from the weird feelings of your body changing its sex. He doesn't reply, and when you look at him, you see he's got an astonished but extremely excited expression on his face. You lean down to kiss him, and as he kisses back eagerly, you feel something really weird, almost primal. Looking into his eyes, you can tell he's feeling it too, and is just as unfamiliar with it.
You're each experiencing the other sex's instinct to breed. Your boyfriend has always had a vagina until just now, so she's never felt the insatiable desire to put her penis inside someone and fill them with her seed. And as someone who's had a penis for all of your life up until now, you've never experienced the feeling of aching to have someone inside you, to fill your vagina and fertilize your womb.
Now you're feeling it, your minds are unable to control it, and your instincts take over. You feel your nipples and clitoris become intensely hard and your pussy moistens rapidly, while his shaft is rapidly back at full mast. Just to underline how totally you have swapped roles, you lie back on the bed and spread your legs, and he gets atop you, rubbing his head against your lips.
He slowly pushes inside you, and your hands cling to the sheets as he spreads your body open. It feels emasculating in the most exhilarating way possible. His length filling you, his firm chest touching your puffy nipples, his balls pressing against your flat crotch, all of it underscores how much of a man he has become and how much of a woman you have become.
As his pace quickens, your hands move to cling to him. He continues to pound you, his conscious mind not controlling his thrusts as nature takes its course. Your own body is bound to his shaft, your walls clenching tightly around him. You're both rapidly approaching your peaks as he thrusts into you faster and faster and you cling to him desparately.
At last you let out a choked moan as you experience your first female orgasm. You feel your whole body pulse with pleasure, the feelings making your casting off of your virility to become nubile seem completely natural. Moments later, you feel him cum, his shaft shooting seed deep inside you, fertilizing you as you cling to his throbbing cock.
You both more or less collapse into each other, exhausted from the intense pleasure, and drift off to sleep.