You give in

The manner in which the strap-on slides through your pussy is unbearable. You obey Gemma’s commands and you give in to your orgasm. Your pussy contracts around the object that’s invading it. You grunt, arch your back and you moan loudly.

Gemma grins with delight at your orgasm. “Yessss, cum for me, ****.”
When your orgasm ends you lie there. Tired, exhausted, but still a girl.
“Ok, so you’re not changing yet.” Gemma states disappointingly. “That’s fine, last time it took you more then two orgasms as well.”

As you try to gather your strengths, Gemma steps off of you. She repositions herself next to the bed and waves the strap-on into your face.

“Suck it, bitch.” She orders you.
You don’t have any strength to resist and do as you are commanded. You accept the strap-on into your mouth and suck it like you would suck a cock.
You can not only taste your pussy juices on that thing, but also trace amounts of Chet's semen. It reminds you that maybe Gemma is right. That you are stuck in your female form and will never turn into a guy again.
That is when you feel the binds holding you to the bed loosen. Gemma is unstrapping you.

“Now, my plaything.” Gemma says to you softly. “I want you to turn your butt to me. Do as your mistress commands.”
Weak willed as you are, you feel you have **** and turn around, facing your rear to Gemma.
Gemma eagerly grabs you by your butt cheeks and shoves strap-on back up your twat.
“Ahhh.” You gasp at the presence of the invader.
“Yeah, bitch. You like that don’t you.” Gemma yells at you as she start pumping into your pussy. “You like that, don’t ya? You little slut.”
Gemma takes you hard from the rear and in your excited state you can’t help but moan loudly.
“Tell me you like it.” Gemma commands you.
“Yes, Yes. I like it so much.” You moan.
As you give in more and more to your lust, you start banging your hips onto Gemma’s strap-on.
“Tell me you are a slut.”
“I’m a slut, mistress. Oooaaahh. I am your slut.”
You can feel your next orgasm getting closer.
Gemma leans over and presses her boobs on your back, then grabs your breasts with both your hands.

You can feel it. The stimulation of Gemma’s breasts on your back and her hands on yours while being pounded furiously, you’re going to orgasm soon. And this next one has enough power to turn you back into a guy … if you’re still able to. But Gemma appears to have realized this as well.
“I see, you’re about to cum again, aren’t you, slut. I can hear it on your breathing.”
She’s right, you realize. You’re heavily gasping for air only to spend it on a loud moan immediately after.
“I want you to say you are a girl.” Gemma orders you. “Say you’re a girl, say it.”
Something in you hesitates. Are you a girl? Or are you a guy? What it your fate?
“Come on, Doe!” Scream for me that you are a girl!” Gemma yells.
“I … I …” you stammer between moans.