Hell, why not?

As you said, it's just a cock! And why not try when the chance is at hands?

"Yeah, but... I've never done this before you know?", you say shyly.

"No problem mate, I'm good with virgins. Just do what all the girls do in the pornos you watch!"

You watched a lot of pornos, so you know exactly what he's talking about. You start of by pouting your lips and kissing the head of his dick. Slightly opening your lips, you let out your tongue and lick around the head of this beautiful cock. You let your lips slide up and down the shaft, teasing the soft skin with your teeth carefully every now and then. You sense that you're pretty good at this, the moans from the other side of the wall are getting louder and deeper with every second.

"You liar, you must've done this a thousand times to be this good!"

You're very happy that you're making a good job. Wait, what are you thinking? You're just doing this because you'll get sucked, too, right? You're not really enjoying sucking cock, are you? This is not the time to consider all this, you've got a job to do. You've already gotten the head of his dick inside your mouth and try to create a vacuum with your cheeks. The moaning lets you know you're doing great!

Since you're a sportsman, you're pretty ambitious, and so you try to get more and more of this delicious cock into your mouth.

"Yeah, slut, take it all the way in! You can do it! You're my little cocksucker slut!"

It's not that you want it, but you're getting even hornier by him calling you a slut and a cocksucker! You wanna prove that you're the best cocksucking slut in the whole world, that there's no other bitch that sucks nearly as good as you! You fight for every inch of cock to get in further, until you feel the head hitting the back of your mouth. You feel a slight gag, but you manage to oppress it, although you have to let the dick out a little for that.

"Oh, was that all you got, little slut? You can't even take it down your throat without gagging? What's the use of a little bitch that can't even deepthroat??"

Oh, your ambitious character! You slurp up your saliva and push your head as close to the wall as possible. Your forehead nearly touches the dirty stall, as the head of the giant monster touches the top of your throat. You suppress another gag, this time without having to back off the cock, and push even deeper. Once the head is past your throat muscles, you feel nothing but bliss! You've done it, you've overcome the limit!

"Yeah you little bitch, this is how you treat a cock! Push it down your whore-mouth and way down your throat, so I can pump that little slut-belly of yours full with my jizz!"

You furiously suck the stranger's member into your throat, enjoying it as it stretches your muscles and goes deeper and deeper.

"Oh yeah, my little bitch, you want Daddy's cum, right?"

A non-understandable mumble is all you can bring out of your filled up mouth.

"All you little sluts want nothing but a belly full of cum and another cock to please! Here it comes, bitch! Suck it all up!"

And with that, you feel his cock twitching in your mouth and throat as it spurts load after load of jizz into your little slut-belly. Wait, slut-belly? What? You barely noticed you shot your own jizz against the bathroom stall from where it drops to the floor as you let go of the cock. It's getting smaller, but you still wonder how you got all that down your throat. And why!

"Well, that was an outstanding performance, I gotta admit! And you really haven't done this before?"

You're to overwhelmed by the feeling of emptiness in your throat and mouth, as well as the questions in your mind. Why didn't you just stick to the deal and jerk it? Why did you have to take it all the way in? Why did you like being called a slut, a bitch, a cocksucker? And why the hell did it make you so horny that you blew your load against the stall wall?

"Well, as I can see, you already had your fun, too, so I guess the deal is done?"