Marcello figured he'd start by using the Change-O-Matic on himself. He successfully got the machine to target his body and a menu popped up with a bunch of options for how to change- species, race, sex, age, all kinds of things. For a moment he wondered which to pick, and decided to go with sex first, since he felt like changing that was most enticing to him. He clicked that setting and set for it to go unnoticed and to change reality, but for him to be aware of the change. Taking a deep breath, he clicked the button to start the change.
An intense energy built in Marcello's body as the device started to reshape his body. Since he had the house to himself, he undressed and looked in a mirror at his naked form. Already his face seemed subtly different, his cheeks softer and rounder, his neck slimmer and its bulge nonexistent. The transformative energy seeped further down, and he felt his shoulders shift inwards and his hips expand outwards, giving him a curved figure. His skin also seemed to shed much of its hair and became smooth. His hands, feet and figure seemed to shrink into themselves, turning less muscular and more lithe. Even without the most dramatic changes having set in, he still looked to be shedding his masculinity, particularly as he realized he now had a large soft butt and higher-pitched voice.
Despite this, two things made Marcello still feel like a man- his genitalia and his mind. Both of these were set upon. He felt a tenderness in his chest and his nipples grew more sensitive. He started to feel them up and gasped at the sensations. The nubs were far wider and protruded prominently, and odd fantasies had started to arise in his head. He pinched and pulled at them, imagining having someone's mouth around them. Soft mounds of flesh swelled under them until soft C-cup breasts had formed, most of the muscle and fat that had vacated him in other places accumulating around them.
The arousal Marcello felt from his new boobs deeply shook his manhood. This new possibility of womanhood felt so good, so enticing. Even if embracing this meant giving up his penis, the capacity to impregnate people, this was impossible to resist! And as he saw his penis start to melt back into his body and felt his balls move up inside him, she saw other enticing alternatives that made it totally willing. She imagined cocks fucking her new passage, shoved between her tits or her lips. She imagined men's warm cum shooting into her mouth or onto her skin or inside her body to impregnate her. Those fantasies flashed through her head- as did her new name, Marcella- until she came, a sensation more intense than any orgasm she'd ever had as her penis finished reshaping into a clitoris and her scrotum receded and split to form a vagina.
Once she came down from the high, Marcella looked at her feminine form. Having a female body felt as natural as having a male body had to Marcello, which surprised her. She wondered what to do with her new self...