Not much voluntarily

To say her pussy quickened before discovering Zack's "me-time" compared to now would equate a meandering river with white water rapids. She gushed with anticipation. She was so close to a boy like this for the first time, and with Zack no less. The star of so many guilty dreams before she slept, curled fetal-like with a hand crushed between her thighs.

And that was... what he had been doing... oh, oh god.

Her legs grew weak, and Madison fell forward, smacking into the couch. She squeaked, more in surprise than in pain, and rolled onto her back. She shook her head back to reality, seeing Zack's mixed expression of alarm and concern.

"Shit, Madison, you okay?!"

"I, uh, fine, yeah." Words weren't being very helpful right now. "I, just... sunscreen?"

Zack blinked. "You sure you're okay?"

"Y-Yeah!" Madison pitched her legs underneath her hips again, but they refused to coordinate. Too much lubrication in between she surmised. "I can find it myself! You seemed hard--" she blushed furiously, "--at work with something! Don't let me break your masturbat--" her swimsuit bottom grew more soaked by the second "--concentration!"

Zack throbbed. Madison's interruption had only distracted his fried brain, not quenched it, and the heat began to reclaim his consciousness. He had usually cursed himself whenever he stole glances at Madison, because the last thing he needed was Brandon riding his ass. He always noticed even the slightest indiscretion among guys around her, and never gave her a chance to respond, unless it was outright rejection. Her reputation as snappish may have been cultivated under her brother's thumb, with no chances to grow into her own.

But now, there was no Brandon, and she had been caught with a worse crime than he ever committed in regards to her. Besides, she turned eighteen a month ago. She was a consenting adult. That made it okay, right?

Madison had somehow stumbled onto her knees and met Zack at eye level. His wanton stare caused her to blend in with the color of her swimsuit. She said she was going to find some sunscreen. The sunscreen wasn't here. She should leave.

"Should" being the operative word. She glanced down at his hardness and twitched.