Make Mark jizz in his board shorts before they can properly begin.

At first, Cynthia plans on toying with the two of them, making Mark get hard and then lose his erection before he can fuck Vanessa properly. After all, of the perks of being in the next room over is that she can micromanage encounters more. She adjusts the settings to give Mark a hard, throbbing erection. Vanessa practically squeals with delight as she feels it grow and harden.
"You're hard!" Vanessa exclaims.
"See? I told you the last few times were a fluke. You're as sexy as ever," he assures her. Holy shit, the last few times? He'd tried to cheat on Cynthia with Vanessa a FEW TIMES since MONDAY? Fuck this, actually. Her blood boils, and in her anger, she lets her previous plans fly out the window. She hits the button labelled "CUM" and leans in close to the vent in order to take in the ensuing soundscape.

"Wait!" gasps Mark, grabbing Vanessa's hand away from his cock. He strains to hold back his orgasm, but it's too much.
"Oh shit," Vanessa says under her breath as Mark ejaculates into his board shorts. It's a short orgasm, and from the sound of it a pretty weak one, but enough to leave a shameful wet spot. There's a long moment of stunned silence before it's broken by Vanessa's voice.
"Hey Mark, what the fuck?" she says, sounding ironically betrayed.
"I, uh..." Mark starts, at a loss.
"I mean, all week you can't get it up, and now you jizz in your pants like you've never been touched before? What's the problem?"
"It's not you. Really, it's not you. I don't know what's going on with me, honestly," his voice wavers.
"Oh, honey, it's okay really," Vanessa says, her tone softening, "I was just really looking forward to feeling you inside me, that's all."
"I was looking forward to it too," says Mark sweetly. Cynthia might throw up.
"Well, obviously," snorts Vanessa, "I mean, you clearly must have been pretty excited."
"Yeah," Mark concedes, "but I don't just look forward to seeing you because I'm horny. You know I love you."
"Yeah, I know," Vanessa says, "I love you too." Oh, great, thinks Cynthia, they LOVE each other? Fuck off.
"Now, I guess we'd better get you cleaned up," chirps Vanessa, and Cynthia hears the two of them begin to move around.