Ah this is a can of worms, fetishes are varied beyond belief!
Personally (so not complaining about anyone elses tastes)
The Mistress has to enjoy doing it, she gets turned on by it!
Sph.....Ridiculing of cock size, that no normal man compete with, comparing to monster sized cocks as, adequate. Leading to.
Cbt.....Punishing cock an balls because she can, various ways, methods, from chastity an getting him horny, to barb wire, scalpels, etc.... An eventually....
Penectomy....Resulting in her most powerful climax! If tortured balls, castration, but enjoy cock amputation in any and all ways possible, but, it has to please her! Angry women are not getting sexual gratification out of it, then its not real submission, more like stress relief than orgasm!