This is, like most fetishes, hard to explain. There might be more than just one reason. Here are some of the explanations:

First, it is all about domination. Most folks into that like to be dominated. They are not only into that kind of play alone, but they also have a foot fetish or they like being trampled or they are submissive. Well, getting shit into your mouth is like the ultimate humiliation, and thus, it gets people sexually aroused.

Secondly, it is erotic when there is something that “comes out of the ass”. Like for girls when there is sperm coming out of a man’s penis. For most men, a woman’s bottom is the most erotic part (even more erotic than breasts). Therefore it is only natural that something that comes out is erotic.

Third possible reason is that scat sex is very intense. In fact, the smell is intense. Everything is intense. And it is a crazy and taboo thing to do it. From early age, you have been told that pooping is into toilets only. So not doing it the “right” way is a turn on as well.

Fourth possible reason is that it is very intimate to receive the bodily fluids of your partner. It is something you don’t share with anyone, thus it can be erotic.

But, like most fetishes, it cannot be explained one hundred percent.

In fact, it might be in the genes, it might result from innocent childhood experiences or something might have triggered it or a combination of the above factors.

The best thing of it is though that no one gets hurt with this play. (unless you practice it unsafe)