With his imagination, of course~

Nathan couldn't stop thinking about Jenna, in sweat-stained, bunched-up spandex clothes, that muscular, well-built man standing over and admiring her, ogling her features as he taught the class, helping her stretch while coping a feel as she pretended to struggle, allowing him to guide her movements into an even more promiscuous pose.

He couldn't stop imagining what would happen when Jenna saw the text, what would happen if she knew she was free to let loose on her newfound toy. Nathan almost felt the smallest bit sorry for Sammy, wondering if he knew what he was getting himself into by hooking up with such a domineering woman. That pity was quickly quashed by an overwhelming wave of jealousy and masochistic ecstasy as Nathan could only wish he was in the assistant's place, being tortured and edged to an overwhelming orgasm by a worked-up, disheveled Jenna.

Nathan hadn't even noticed his hands having drifted from his sides to his crotch, instinctively pulling at his loose pants, which gave access to his boxers with ease. Off the undergarments went in turn, pooling at his ankles as he delved back into a horrible, wonderful, oh-so-wrong daydream.

Grasping at his fully erect length, it was quite easy to complete a full stroke, both hands being unable to even grasp the length together without completely overlapping, leaving his non-dominant left hand fondle his tip. He wondered just how big Sammy was, how easy would it be for Jenna to wrap her slender, skilled hands around his huge cock, to utilize the incredibly pleasureful two-handed techniques she's shown Nathan but never used on him, always teasing him with the same false innocence, her taunting, sultry words ringing through his head; "Oops, I guess you're too small to use this one on... maybe I'd be better off getting a dildo to practice on instead~?"

Though now she seemingly had something, or rather someone, better than a dildo to tease with her skilled hands, to rile up and edge over and over, before even beginning the act fully.

Then his mind finally settled back on that horrible idea, the one that made his stomach churn and his mind go numb with rose-tinted jealousy. Jenna would be having sex with another man, with a bigger cock, while in a state of arousal and excitement surely equal to Nathan's own, all while coated in sweat and pent-up lust from who knows how long ago. It never even crossed Nathan's mind before that ever since that first day they went to yoga lessons, the only reason Jenna had stayed was for Sammy. The very thought made him quiver with a depraved sense of envy and lust.

Finally, he felt himself nearing the edge, something he had been taught to be very aware of by Jenna through rigorous "training" to extend his stamina in bed. Finally, he allowed himself to imagine Jenna, all ruses of dominatrix behavior shattered, being fucked silly, legs spread wide above her head, clothes displaced and hanging loosely off of her shoulders and ankles, as her womb was claimed anew by a larger, more pleasureful black cock that she had never known before, as this imaginary version of Sammy looked back at Nathan, smugly smiling in both pleasure and victory, a look that drove the immensely horny boyfriend over the edge.

Just as he came, his shameful dick spurting out the evidence of his newfound fetish, his phone went off. He barely noticed it at first, his mind still in the afterglow of his intense orgasm, but he soon noted the glow of the screen beside him, and he picked it up, seeing what Jenna had to say.

For a moment, he still clung to the hope that Jenna was just teasing him, that he could admit to this naughty behavior and be punished without Sammy ever getting to see his girlfriend in such a way, disheveled and half-naked, another man's cum dripping out of her stretched, pleasured pussy.

But at the same time, a sense of cruel masochism drifted over his mind, desperately hoping for the "bad" news that Jenna would be using the go-ahead to drag Sammy to some back room of the class building and let him use her as he pleased... or more like use him as she pleased, knowing Jenna.