If the government really wanted to, it could provide citizens with a portal that would do oauth (or something similar) to authorize the porn access.
They could do some crypto crap to avoid storing anything about the citizen, so, unless the system is subborned, it doesn't store anything about users.
EDIT: the point is that this kind of system can be implemented in a privacy-preserving manner. I'm ambivalent about the idea, but it has been enacted by a democratically elected government, so they should go about it in the most responsible manner possible.