Can be a couple reasons... For one, the mask might play into the random stranger/rape fantasy/sense of danger element if the porn is taking that angle. But also, far out kink can get judged pretty harshly. Some people don't want their personal identities associated with it out of fear of blowback. A lot of women who do amateur porn wear masks for the same reason.
gues it's a part of the BDSM "culture" obviously...:-)
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The notion of the executioner A. K. A. the ultimate punisher.
Maybe it's just a thing. Or maybe it's so the person who's not wearing the mask doesn't know who's on the other side.
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