...a white underwear that belongs to a guy from the closet. This underwear is only fit for a big boy and it's not gotta fit for a girl, but you know it's not gotta fit on your body. As for your girlfriend, she might fit in that uniform, plus she's always wanted to be a boy and wonder herself what is like to be a boy. And honestly, one time you accidentally caught your girlfriend wearing men's clothing after you entered the room. She was shy and embarrassed after getting caught by you, but you're not laughing at her or anything. You're okay with her having her own kinky. And even during sex, your girlfriend dresses up as a boy for roleplaying and thought it would be a fantasy gay sex and fetish. Now, she's about to try it out just to see if the transformation would work. First, you watch her take off her clothes and throw them on the floor as she reveals her perfectly naked feminine body. Your girlfriend can tell you are enjoying the view as she smiles at you, and then she takes the boy's underwear. Of course, it is easy to fit on between her thigh, but it's too big for her butt. "Damn, I didn't know this underwear is big for my body," she responds.
"If you think it's large for you, you can just try different clothes, you know."
"I know, honey. But it would be interesting for us if we have a role-playing game with this." Wow, didn't know your girlfriend was that kinky. After she puts on her underwear, she suddenly starts to feel strange around her body. She starts to feel hair around her body, especially her face, body, and legs. Then her face starts reforming more muscular and boyish as her long hair begins to grow back to a short boyish style. Then her breasts start shrinking, and her stomach starts forming a six-pack as her light skin tone is more tanned. Then her clitoris starts growing and forming into a penis. Finally, her arms, legs, hands, and feet grow more muscular. After the transformation finishes, she, or rather he, now goes and has a look at his mirror to examine his new body. "Wow, look at me, I'm jacked." Your former girlfriend, or now boyfriend, likes her/his new body.
"Of course, you are, babe. You look quite dashing." You reply.
"Thank you, hun." S/he says back to you. "Oh and by the way, if I'm now a guy in this form, can we both address each other as dude, bro, or man like the boys usually say each other?"
"Alright, I see what you mean, ba... bro." You begin to understand what your BOYfriend wants to refer to. "Anyway, 'dude', what would you like to do with your new body?"