Defecation onto one's self by (or onto) another, and urination onto one's self (or another), well, that's not at all "on my radar screen," but for some it's a turn on.
I'll never really understand their amusement with it, but as we say, "to each their own." lol I think it's a power play thing, an humiliation thing, and a pretty twisted element of "sex." But so is auto/mutual erotic asphyxia (strangling and choking), and some people do it all the time.
It's fringe activity, and very, very hardcore. It just does NOTHING for me... BLECH