Biggest load pays

"Here's my proposition. Just a little friendly competition I enjoy to make things more fair. Basically, we all finish our drinks here, pop over to the ladies room, and jerk off into our glasses. Smallest load has to buy the next round." Astrid said rather boldly. Sarina was taken aback, but tried not to show it, merely raising an eyebrow. She thought it over. Her prosthetic member could technically ejaculate and she was sure it could put out quite a bit, as the package had explained that with the balls fully filled with the proper fluid, it would be able to functionally ejaculate roughly 10 times before needing a refill. The trouble was she wasn't sure how long it would take to make the thing finish, especially just jerking it herself. But she'd just gotten the damn thing, might as well test it out, right? She eyed up Astrid and Ashley again. Astrid was clearly the dominant one of the two, and a serious tomboy. She had the same tight shorts on as Ashley, but where Ashley sported a flattering tight sweater up top, Astrid wore a rather raggedy ACDC tank top. Glancing down at their bulges, now that she could get a closer look, it appeared that for all her bravado, Astrid actually had a rather medium sized bulge compared to her sister, who looked like she could give Sarina a run for her money.

"You know what? Why the hell not?" Sarina replied, before downing her whiskey in one go. It burned, but she managed not to cough. "Hell yeah!" Astrid shouted, while Ashley blushed even harder. To her credit however, she managed to finish her drink almost as quickly as Astrid, and followed willingly along to the bathroom with them.

As they pushed through the door to the bathroom, Sarina noticed something that made her nervous. There was only one stall, next to a wall line with urinals. The stall was....