"Well, I have an idea," you smile. "That looked pretty cool, so... I want a turn."
You wish to become a woman, and quickly undress, stepping in front of the mirror to see your naked male form one more time before the change.
As it starts, your breathing quickens and you feel weird pleasurable shivers run through you. Like her, your figure is the first thing to transform, as your shoulders narrow and your hips flare, your butt expanding. You feel your girlfriend's eyes on it as you become more shapely. Your arms and legs, and hands and feet, seem to decrease in size, your figure more petite and elegant than strong and firm.
Just as her voice deepened, yours turns higher-pitched as your larynx shrinks until it isn't visible in your throat anymore. Giggling, you notice your new voice is pretty cute. The surges of pleasure, which you've realized must be from feminine hormones, are growing stronger all the time.
You see your body hair vanish aside from on your head and crotch, making you look smooth all over and much less masculine. Even your skin seems slightly softer and smoother now, and little twitches in your facial muscles make you notice your face turning more feminine.
At last your genitalia starts to be changed, and the pleasure intensifies even more. You moan as your nipples perk up, suddenly feeling a million times more sensitive than before. They widen and swell, and as you breathe, mounds of flesh form around them, enlarging until you have perky D-cup breasts. Cupping them, you're surprised how warm and soft they are.
Looking down, you feel the last change affect you. Your testicles are tugged up inside you to form your ovaries, the space where they were tucks inwards until your crotch is almost flat, and your cock rapidly shrinks into you, the nerve endings concentrating around the tiny nub that remains. Once everything else has finished, a slit opens between your legs, a new deep passage to be fucked within you.
You turn and wink at your girlfriend, and give him a come-hither gesture.