This may be a nitpick, but it seems to me that scat isn’t that closely related to most forms of BDSM. At the very least it isn’t closely integrated with mainstream BDSM. For example, the FAQ for the SSBB newsgroup says “Some people are into scat play, which is playing with shit. I don’t know any who are, but they’re out there.”, and while my experience with “the bondage community” is limited, I’d guess that bringing up the subject of scat play at a BDSM party would get someone about the same kind of response that a “cat o’ nine tails vs. bullwhip” discussion might get in a more mainstream crowd. I understand that anal fisting (apparently more common among kinky gays than kinky straights) can get a little messy, but the attitude towards the… uh, mess seems to be that it it just something that comes with the practice, and it isn’t a focal point of the act in the way that a paraphilia would be.
Beyond that, I have no idea how people become aroused by scat.