Types of D/s relationships, in order of frequency:

-Master/female slave

-Master/male slave

-Mistress/male slave

-Mistress/female slave

I think this is biological, plain and simple. So, you all men and women looking for mistresses, don't be disgruntled if you never find real ones. Especially female subs like me: just forget it. Also, keep in mind the "mistress" figure is mainly a male invention, modeled on his own desires and sexuality, and seconded by smart women who make a living out of this. The whole "mistress" thing is nothing but a gimmick to reap the gullible. Good luck!