This could be very easily done at the ISP level for the whole connection. (I'm talking best effort not a perfect implementation)
Why not just block all "Adult" content by default and allow the buyer to opt in (both temporarily via password, and permanently).
Then it would be a decision each household could easily make about what they want.
edit: I love the downvotes without rebuttal. Reminder that the downvote in this community is not "I disagree with you"