Go into the ladies' and look for the larger cubicle

You do as he says, and walk into the ladies' bathroom. There's a few other women in here, taking turns at the cubicles and others hanging over the sink. None of the cubicles look larger than the others though, so you push past a few of the other people here until you reach the end of the line of cubicles. And aha! That one on the end does look larger. You push at the door, and it swings open. Glancing back towards the entrance of the bathroom, you see a couple of the women are now looking at you with knowing expressions. What they know however, is a mystery to you.

You push into the cubicle. The door swings shut behind you, and you hear it lock with a click. The room is dimly lit, and sealed off from the rest of the bathroom. Your eyes start to adjust, and you can make out a hole in the wall, opposite the entrance. Then, as you get more used to the dim light, you can see a bench like shape in front of the hole, level with the bottom of it. You notice two smaller holes above the main large one too.

It kind of looks like- no, that's a silly idea. But... maybe if you were to lie on the bench-like object, you could get your feet in those smaller holes? And then that would mean- that bigger one is for your ass...