Alright, I might be able to explain this one. There are a few reasons for it but what I think you're missing is that it's not really a fetish in and of itself, it's a combination of fetishes. If someone was into having a woman put their feet into their face while they were tied down it would be a clear combination of bdsm and a foot fetish, and in that way it would be easy to see how the combination would arise. In genital mutilation this is less obvious since it's often treated like it's own thing.
One part of is is a sexual attraction to pain. This is actually a lot more easily understood than most people think. When you're aroused the body doesn't feel pain with anywhere near the same intensity, as well when you experience pain your body sends out all kinds of chemicals, and some of those chemicals feel pretty damn good. So in a sexual situation the positive feelings caused by endorphins can often outweigh the physical aversion to pain. This is especially the case when you're someone like myself who lacks to a large degree any kind of sensitivity. (an infection post circumsicion became infected with fournier gangrene, as a result I have very little sensitivity in my bits.) The more pain, the more endorphins, the better the feeling afterward. It leads to a kind of feedback were pain is in and of itself sexually appealing. The other part of it is more or less standard to extreme bdsm affair. The enjoyment of giving someone control over you, of helplessness or just of the tactile pleasure of being bound. When you combine the two you start seeing things like this chastity becomes emasulation, when you're bound you fantasize about being tortured. Really the submissive partner is still in control, and everything is (or should be) consensual. It's just that the kinds of consensual play begin to skew more toward things which lead to pain.
