I know that at least one dominatrix directory I advertise on is changing its photo policy to no nude, no topless, no extreme acts etc etc just lingerie/clothed photos and some other restrictions, so they can try and avoid needing viewers to their site to sign in with IDs etc.
I would guess others, like Adultwork, that aren't just directories but adult content sites will have to initiate ID checks. I don't know what that will do to their traffic, I imagine it'll take a hit of some kind, but how much, piece of string etc.