As long as it's being done by consenting adults, no problem. My problem with the porn representation of this is that many of the women involved are doing it solely because they are being paid for it, not because they enjoy it, even remotely. (And, yes, there are a number of porn performers who do enjoy face fucking... Sasha Grey comes to mind... but that's more the exception.)
This is one of those activities where you really want to discuss your expectations and guidelines beforehand.
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