Over her back

Phil remembered the chances of knocking Alex up was high, and knew he couldn't risk it despite the urgency of his orgasm and that ultimate taboo. He quickly pulled his cock out of her pussy the moment he felt the first pulse of his orgasm, and his cum spurted up her naked back. Jay saw it, groaned, and tilted his head back, his hands on either side of Alex's head holding her down on his cock. If Phil had been aware enough to notice, he would have known Jay was cumming down his own granddaughter's throat, but Phil was too focused on his own orgasm to notice anything.

Hands tight against the teen's hips, pressing his throbbing cock against the crease of Alex's ass, his cum pumping on her back again and again. He couldn't remember the last time he came this much, or even the last time he had been this excited. His eyes roamed from the curves of her ass, up her cum painted back, then back to his cock laying against her skin.

Then Alex coughed, and Phil realized Jay had let her head go. He looked up at the older man, who's head was laid back against the wall, his eyes closed. Phil leaned forward, his hands sliding up her body and under, grasping her large breasts to pull her up against him. Alex was still gasping, wiping her mouth with her hands, almost unaware of her dad pulling her up until Phil started whispering in her ear. "I don't want you to do this with Grandpa Jay anymore, understand?"

Alex looked at her father's face, inches from her own, and nodded.

"And we're going to get you on birth control so I can cum inside you next time."

The look on Alex's face showed clearer than words that she had never expected her dad to ever want to fuck her, let alone do it more than once. Phil kissed her mouth, pressing his hips forward, his cock against her back, another drop of cum oozing from the tip and onto her skin.