This explaination is a nice start but I'd like to read it from a larger perspective.
Why scat can be arousing? I think it's something on the side of the relation between master and servant.
So, at first, what does it make the master-servant relation biologically acceptable? Given the existence of some kind of social structure (which is pretty the case) the least gifted individuals are more likely to survive if they live under a mighty master, rather than by their own. In a few words, it's better to be ruled than preyed.
Then, a lot of animals are used to mark their territories with excretions and body fluids. You surely know how dogs tend to pee and poo in the same spots. And here is the second ingredient of this strange recipe! An individual who shits nonchalantly everywhere could give the impression of being a worthwile master.
The subject can also be looked at from the point of view of the master because those who shit looks harmless and helpless, the perfect pray! Quite simple this time, huh?
Now, nobody is born master or servant but we all have these circuitry, ready to be switched on and linked here and there. And since we're talking about relations among individuals, this can surely affect the sexual sphere. How? The hell I know! However the very first question was "why", wasn't it? :P
Whether this may be considered socially right or wrong is a matter of... contingency.
Beside, I am not a scat lover... usually... XD