Pick the already occupied booth

Why not? Since the door is left open, it's most likely someone who wants people to join him. You walk towards the booth with the red 'occupied' light and take a peek inside.

"Hey, come on in", you hear someone say, before you'd have time to adjust to the darker lighting inside of the booth. You step inside and take a look around.

On the side walls of the booth, there is one hole on each. Both conveniently at waist level. On the back wall there's a small TV screen playing just some porn. Next to it there are instructions on how to change to a different video. Sitting on a small stool you see a man, stroking to a porn video playing on the screen.

"Don't be shy, come closer", the man says. You take a good look at him and try your best to hide your disappointment: He's old, probably at least 60, and seriously overweight. You gulp. It'd probably been better not to know what he looks like.

"Come here, I have something for you", he says stroking his short dick. It doesn't look inviting at all.