Keep Mark from getting hard long enough to fuck her properly

Cynthia adjusts the settings in order to allow Mark a weak half-erection. She wants him to have hope that he's going to be able to fuck Vanessa. She also realizes that being in the next room over means that she can micromanage the encounter in a way that she can't when she and Mark have sex. She lets his cock get harder.
"You're hard!" Vanessa notes with delight.
"See? I told you the last few times were a fluke. You're as sexy as ever," he says. Holy shit, the last few times? He'd tried to cheat on Cynthia with Vanessa a FEW TIMES since MONDAY? Her blood boils as she taps her phone, causing Mark to lose his erection. A few moments later, she hears Vanessa's voice again.
"Oh. What happened?" she asks.
"Uuh, nothing," he replies, clearly embarrassed, "I swear, it's not like the last few times. I can get hard, I just, uh..."
"I could go down on you a bit?" Vanessa offers.
"Yes! I'm sure that would help!" he eagerly accepts. Cynthia hears the sound of board shorts being pulled down followed by the faint wet slurping of her best friend's mouth around her husband's limp dick. Cynthia slowly adjusts the slider to allow him a semi, and while she's there, she turns up semen production on a whim.
Mark moans, leading Cynthia to double-check that orgasms were disabled for now in case she'd forgotten. They were.
"I'm so wet," says Vanessa eagerly. So am I, realizes Cynthia, sliding her bikini bottoms down around her ankles before her pussy can cause a wet spot. She absentmindedly strokes her clit with one hand, operating her phone with the other.

After a few minutes of Mark getting hard and then soft again in Vanessa's mouth, Cynthia decides she'd better finish things up if she doesn't want Vanessa to get too frustrated and leave. She gives Mark his long-awaited full erection. As expected, Mark's recent habit of premature ejaculation is present enough in his mind that after about 20 seconds of his cock being hard, he removes Vanessa's mouth from his shaft.
"Alright, I'm ready," he breathes, "I want to be inside you now." Cynthia hears shifting, followed by a high-pitched gasp from Vanessa and a grunt from Mark as he enters her. Her own orgasm mounting, Cynthia disables Mark's erection once again and listens with a sly grin as he continues desperately trying to fuck Vanessa as his cock deflates.
"Are you for real right now?" asks Vanessa, clearly frustrated and annoyed. Cynthia grins from ear to ear as she listens to the sweet sound of Mark's flaccid cock being withdrawn from Vanessa's wet hole. She can tell that Mark's stroking his cock frantically, trying to get hard. It's all she can do to keep from gasping and moaning as she taps the button labelled "CUM". She hears a panicked vocalization from Mark followed by a shocked gasp from Vanessa. As Cynthia reaches her orgasm, the excesses of cum which had been brewing in her husband's balls at Cynthia's command erupt from his soft cock and land, if the "Shit! Mark, turn away!" she hears is any indication, all over Vanessa.

It's hard not to laugh as she hears them scramble to minimize the mess - after all, they can't come right out and tell their hosts that they got themselves and the duvet of one of the guest beds drenched in cum while Mark was carrying out an affair.
"Okay, you know what?" Mark's voice can be heard saying, "You take a shower. I'll put the bedding in the laundry and tell everyone that you were drunk and were feeling a bit sick so you went to lie down, but you ended up throwing up all over yourself and the bed. I was walking past, heard you in being sick and got worried and came in."
"Gross," says Vanessa unenthusiastically.
"Exactly, it's gross!" says Mark, "Not to mention embarrassing, so people won't ask too many questions."
They continue discussing it, but Cynthia's heard enough. She pulls her bikini bottoms up, washes her hands, and slips back out into the party. Nobody had even noticed her absence.