"Tell me what you're thinking about."

Mark's voice was a low grunt in her ear. He was behind her, his weight a familiar, heavy pressure, his cock moving inside her with a slow, steady rhythm. They were in their bed, the room dark except for the sliver of moonlight through the blinds.

Anna hesitated, gripping the sheets. The fantasy was new, a dangerous little spark she'd been nursing for weeks. It felt stupid to say it out loud.

"Come on," Mark urged, his thrusts a little deeper.

"It's that look you get. Your mind is somewhere else. Tell me."

She took a breath.

"What if... what if there was someone...