I think there's no excuse. Even without access to a bath or a shower, a sink of hot soapy water and flannel is also perfectly possible to get your ass clean.
Imho, there is NO EXCUSE WHATSOEVER. It's just down to lazyiness and a total lack of effort and a total lack of having the will to ensure that you are squeaky clean. Even with just a flannel I can come out of the bathroom totally clean and fresh - it's the resolve you use (scrub it good!) and the technique (as someone else said, you've got to get into the crevices, open your legs properly, and do a proper thorough job). That takes care of cleaning the outside which ANYONE should be able to do.
In terms of the inside as I said before there is no way you can engage in anal play without it being a disgusting smelly shitty event unless you clean out the inside too. That involves going to the toilet to empty the bowel and then using an enema where you squirt the water right up your ass and poo out ALL the remnants of poo that are left in there. THEN you are ready to play.
So even in the case of someone who is so overcome with desire that they lose control of their anal sphincter during the act - that too is irrelevant because if the inside was clean, again, nothing would come out would it!!
As for the topic of what do you do if they come to you like that. Well, whether it would cause me to labelled as an impatient inconsiderate cowbag, if a guy came to me smelling of shit with it stuck to his ass and pubic hairs I'd throw his ******* ass out the door and he'd be told in no uncertain terms that it was not acceptable. If animals can keep themselves clean, so can a fully functioning human being with two hands and access to soap, water and flannels. There is simply NO excuse and why anyone would put up with that is beyond me (that's just me lol!). No amount of money is worth getting close to someone elses faeces who doesn't have an actual medical problem that causes leaking of this stuff (in the which case, one would hope that the client would advise this first and you would have the choice of whether to see them or not and chance to work out how to deal with it by getting as much information as possible in advance).
But these guys that are coming with dirty back sacks, cracks and rectums are just lazy, dirty gits (again, in my humble opinion) and I wouldn't even deal with that with gloves on. Sorry. Why should we speak kindly and softly to a person who hasn't got the respect for themselves - or for you - to even wash themselves properly when they come to see you.