I'd say porn is an exception simply because it hijacks people's attention, and therefore the view count doesn't represent what value is derived from it. You'd just be counteracting the disproportionate amount of views those videos get. Presumably you don't want to shape the traffic because you want creative content to thrive. You're only making more room for creative content by filtering out the porn. That's my two cents.

It's important to focus on the casual visitor who doesn't have an account and doesn't actively filter even if it's based on cookies. Solutions 2-4 don't work for them. Solution 5 sounds like it's part of solution 1.