So I am enjoying the similarities between your depiction of this transformation and my fantasy :). Please carry on the great work you are doing!
My crush's ejaculation was so violently powerful that with the first spurt i felt it filling my entire vaginal cavity, but I saw her testicles immediately getting sucked back into her returning to ovaries. My clitoris immediately swelled and I felt blood rushing to it as it grew at an alarming rate, just like her breasts and female features. Before i knew it I was exploding again as a male with her holding my returned rod. Both looking at each other bewildered and further enamored by our deeply intimate knowledge of exactly how to pleasure each others bodies. The look in our eyes, a promise of the experimentation yet to come, and our yearning for it.
The fantasy would carry on to the reverse happening whereby I'd get to explore all of the secrets of the female body and give into them as over time we taught each other (my transformed crush and I) the intricacies of our swapped genders. The culmination was as we brought each other to a final orgasm within each other and our bodies would begin to revert much faster than the initial transformation, as if our bodies yearned to return to their original form after this enriched experience. The orgasms would hit the point of no return, our hands clenched each other and our bodies tensed as orgasm exploded between us.
Simultaneously the retraction of my testies progressed into my body with the vaginal opening slowly forming both externally and internally, new sensations catching me by surprise causing me to flex my tender leg muscles and as I did I could feel them filling out, my bum filling out into a round shape along my new broadening hips. Slowly my breasts kept growing, filling the palms of my hands giving me something to grasp. They kept expanding 'till there was more than the span of my now elegant hands could hold.
The ejaculations stopped as the connection between testies and penis began to rearrange itself, however the pulses and convulsions of orgasms did not stop. A warm radiance kept building all across my skin as my bones gently, yet unmistakably, rearranged themselves modifying the shape and contours of my body, my skin softening and my voice heightening in pitch moan after moan. Breasts began to grow in a series of sped up hormonal fluctuations, just like I'd seen in sex-ed and read in books about how the body grows, with the nipples first bulging out followed by the growth of the mammory tissue, soreness alternating with tenderness that I could not help but cup and feel.
At the moment of orgasm I would ejaculate incessantly with each gush drawing out more from me with breathtakingly strong pulses travelling through my body. The brief moments in-between the waves of sensations rushing up and down my spine I could feel my penis throb as though it were absorbing something in return from my partner. Looking closely at our bodies and the sensation of my throb shrinking in size but not hardness, I quickly realised the exchange of fluids marked the swapping of our genders. The most immediate change being initially in the genitals where the penis shrank and her clitoris began to swell. Suddenly I would find myself with nothing more than a bump between my legs, no longer enough to be within her.
I really enjoy the twist you are putting to this transformation as its very similar to something I used to imagine quite a lot. When I was back in my teens and very curious about what the other half felt during sex one of my most strikingly pleasurable fanatsies involved myself and my crush at the time in a sort of cocoon/womb environment both naked. We'd start pleasuring each other in several ways, the twist being that neither of us knew that at the moment of orgasm was when things would trigger.