You look around in disbelief as you start to shrink. Yeah, this wasn't a dream. You really were being de-aged to a sperm and egg! You don't just seem to be getting younger, though, you seem to be physically regressing. Your limbs seem less developed than they should be at each age, and you feel like you're being pried apart by some force from the technology. You feel your body lose its form and shape, though weirdly you're entirely conscious through all of this, and you scream in confusion and fear, your voice turning higher-pitched and younger the whole time.
At last, you're a pile of juices on the floor, somehow able to see and hear everything even though you don't have any organs with which to do so. "OK, so now we've regressed him and divided his genes so that the ones from the sperm and ovum are separate, he's going to be split between Mina and Steve," the teacher explains, pressing another switch.
When he does that, your consciousness seems to split into separate simultaneous planes each of which you can see, like splitscreen in a video game. On the one hand, you see and feel a part of yourself being drawn inside Steve's body, and on the screen more sperm accumulate in his balls in real time.
Another part of you is split off and moves into Mina's vagina, with your ovum form showing up inside her on the screen.
Finally, you find you have a great view of the class somehow, and can feel how excited and aroused the machine has made both Mina and Steve.
"OK, you two," the teacher smiles at them, "I think you can do this next part instinctively." Encouraged by the pheremones that filled them when your genetic material moved inside them, Steve and Mina start kissing passionately. When Steve pulls off Mina's shirt and bra and starts sucking on her nipple, you feel the same surge of pleasure she does, and when her hand reaches into his shorts to rub his cock you enjoy the excitement he gets in tandem with him.
A few seconds later, they've pulled each other's clothes off and Mina is lying back on the floor naked. Steve starts thrusting in and out of her eagerly and both of them moan. Your mind starts going blank from the simultaneous male and female pleasure, their feelings of arousal and joy combining and accumulating in your split body. And just to accentuate that, you enjoy the additional thrill of knowing your body was about to combine again.
Mina cums first, and as her pussy spasms around Steve's hard cock, the ovum part of you shares her intense female orgasm, something you've never experienced. All of a sudden that part of you is overcome with a desire to be fertilized, disconnected from you specifically.
As Steve hits his own peak and ejaculates, shooting your sperm forcibly from him, the sperm part of you realizes something: it's not guaranteed to get to the ovum before any of Steve's seed. And it turns out he has strong swimmers...