"Have you thought about changing genders?"
Ty blushed. He should've figured Gavin would come up with something like that. But... it did sound kinda fun. He had always wondered what it was like to be a girl, he wondered how Gavin had figured that out (if he had). "I... I guess I could try that, yeah."
"Nice," Gavin grinned. "Now take off your clothes!"
"Dude!" Ty yelped.
"Do you want to rip them when you transform?" Gavin pointed out. Ty hated to admit it, but that was a fair point. So he pulled off his T-shirt, tugged off his jeans and stood in front of Gavin in his underwear.
Ty focused on the idea of changing genders, hard, until he felt a tingling sensation start to spread through his body. It felt way more intense than he'd imagined, as masculine hormones became feminine hormones and his skin rippled flexibly as it began reshaping.
His torso sloped inwards as he formed curvy hips and a narrower waist, and his skin softened all over and most of his body hair vanished. He winced as the changes spread through his face, and as he managed to get his eyes open again, he heard Gavin say, 'whoa, you're hot!' Picking up his phone to see his reflection, he realized his facial structure had altered- and when he gasped, he noticed his higher-pitched voice.
The changes then moved to affect his reproductive system, and he felt still more conscious of Gavin's eyes on him. His nipples became sensitive, swelled up and got erect, and loud fast pants came from him as soft flesh flowed into his chest. He saw the flatness of his chest give way as he sprouted soft breasts, and instinctually covered them from Gavin's pervvy eyes.
He laughed as he noticed what was happening between Ty's legs. "You're so hard, dude!"
"Wha-ahh~" Ty moaned as he felt the sensations spread down his stomach to his crotch, dropping his pants to relieve his throbbing erection. He barely got to stroke it before he came messily, his seed squirting onto the floor- at which point the feminine hormones inside him had their way with his manhood.
Ty's brain was barely able to comprehend the changes as he became castrated, his testicles moving up inside her to be claimed by her freshly formed womb as her ovaries, his scrotum tucking inward and unsealing down the middle to form her vagina, and his penis receding into the tiny sensitive nub of her clitoris.
Once they had all formed, however, she felt her femininity click for her instantly. She was a girl now, and she liked it better than she had liked being a boy.
...a moment later, she noticed Gavin staring speechless at her with a rock-hard boner.