On the face of anyone they can overpower (poop later on)

In this world, people are the 'toilets'. But it is incredibly shameful to be on the receiving end of a faceful of piss or shit, and so you will meet and resistance if you try.

However, the worst thing of all is to be caught doing your business somewhere except on another person. If this happens, you can expect a mob to form and implement their own form of justice, which will likely involve you being -fed a whole lot of excrement. (In general you wouldn't necessarily be expected to swallow urine, and you definitely wouldn't be expected to swallow feces as it can be bad for you, but if you're caught disobeying the rules of society, then you'll probably end up in this sort of trouble. Hence, it hardly ever happens.) There is an exception - if it wasn't your fault. For example, if you're left tied to a chair, nobody can really blame you for wetting yourself. Also, people who have an extremely strong bond of trust and a morally grey code might keep it a secret even if they find out. This would be like hitting someone with your car and keeping on going without checking whether they're ok, and the person in the passenger seat agreeing to never mention it again.

So, I hear you ask: If the recipient of the excrement is going to be attempting to escape, then how do the weak and scrawny people of the world manage to successfully use the 'toilet'?!

Well, firstly, people don't go around on the alert all the time - that would be too exhausting. So it's easy to catch people unawares, and they'll only start struggling once the has begun. And furthermore, there's a few options available to give yourself an unfair advantage.

Firstly, there's power in numbers. All you need to do is bring a friend or two to help you subdue an unsuspecting victim while you do your business on them.

Secondly, there's ways for small people to get the upper hand. Slipping a pair of handcuffs on someone before they can react could give you an advantage over someone, as could simply sneaking up on someone who's asleep. There's also several items of furniture and tools specifically designed to allow you to temporarily overpower someone, as long as they're not extremely bigger or stronger than you. For example, there's a sort of modern cross between a lasso and bolas which you can toss at someone's ankles to wrap them up in rope and cause them to fall to the ground.

Thirdly, someone who is very very affectionate towards you might let them use you as a toilet in private, and endure the shame out of their love for you.

Lastly, as with everything in a capitalist society, there's going to people willing to endure the awful humiliation of letting you use them as a toilet in exchange for cold hard cash.