She makes you fuck Cassie, with a strap-on

"I want to see how the two of you perform, and behave. So I want you to fuck. Go on, get up on the bed, but dont you dare touch each other until I say so!"

You and Cassie struggle to get up, and barely manage to get on the bed, but you make it eventually. Now, from up close, you can see that your girlfriends pussy is dripping wet. You feel a light sting-jealousy? Was she this wet because of Nyx? Then again, you´re still rock-hard as well, just by being bossed around by her ex-girlfriend, so maybe she was thinking the same.
Still handcuffed, naked, you looked at each other helplessly while waiting for Nyx´s next command. You dont dare to speak without her permission.
You see her grabbing something, and your heart stops as you see what it is.
Nyx carries a large strap on with a belt attached to it.
She laughs at the look on your face. "Awww poor baby. Did you really think you were going to fuck your girlfriend with your real dick? Dont be silly.

You can do nothing but watch as she attaches the strap-on above your real dick.

Your dick isnt that small, but the strap-on is at least 2 or 3 inches larger than you, maybe even more. Your dick looks pathetic hanging under it. Cassie looks at the strap-on and bites her lip. She looks scared by the size of the fake cock, but also...exited?
You feel yourself getting red as both the women take a moment to look at you.

"Really puts things into perspective, mmh?" Nyx asks Cassie. "Now, get on all fours."

Moments later, you are positioned behind your girlfriend, her ass stretched towards you. With her hands still cuffed on her back, her face lay down pressed against the pillow, with no other means of supporting herself.

"If any of you dare to cum before I tell you to..IF I tell you to, it will have consequences."

Then, the domina comes closer, and kneels down next to the two of you. She grabs the fake cock attached to you, and slowly guides it againt the lips of Cassies pussy. "Now show me how you fuck your girlfriend." She smiles.
Best as you can with your hands cuffed, you thrust forward.
Immediately Cassie yells out, something between pain and pleasure. You stop, but Nyx shake her head. "Keep going. I know she can take much more than that. Cant you, Cassie?"

Cassie doesnt answer for a moment, but then you hear her say: "I can take more."
So, on Nyx´s command, you begin to fuck her. As you pick up the pace, stretching her pussy out more than you could with your own dick, your girlfriends moans get louder. You watch inch for inch of the strap-on dissapear into her pussy, while your own cock slaps against her and your legs uselessly.

Still, it turns you on much more then you thought. The small stimulation of your dick slapping agains skin, together with your girlfriends moans, your helplessness, and the sight of the dildo going in and out of your pussy make your cock drip with pre-cum.

After some time, in which Nyx watched the two of you, she nods. "Time for another step."