They can't feasibly sift through all websites and categorise them, legally, as porn or not porn. Instead they use filters which identify proxy material that may be indicative of pornographic content.
Therefore it doesn't really matter what the legal definition of porn is.
The result will be thousands of perfectly innocuous websites (and borderline cases) being blocked by default because they feature text that some panel of software designers feel may be associated with porn. My work employs such a filter and it blocks this entire forum (I guess because of the N&G subforum). It also, ironically, blocked some news stories about this very subject. And it blocks perfectly inoffensive websites offering advice on sexual health, family planning, and the like.
So I wouldn't rest on the "legal definition" of porn. It's not how this stuff works.
That said, it should be easy to opt in. You'll just end up on a de facto list of people who have opted in to view pornography (or, at least, that's how it will be viewed) which will inevitably be leaked at some point down the line. Or you could just browse through a proxy.
Stupid, pointless law.