"In 1964, Justice Potter Stewart tried to explain "hard-core" pornography, or what is obscene, by saying, "I shall not today attempt further to define the kinds of material I understand to be embraced . . . [b]ut I know it when I see it"
To an extent that sums it up.
Mature sites are removed from listings. As soon as we state that a Mature site is defined by A B and C then those running those blogs will move 'inside' those rules so they can get listed.
That's one part.
We do not want to people to find themselves inadvertantly visting a blog featuring adult content. People browse here from work, people help here while at work and the last thing needed in a browser cache is porn (note - a boss WILL call it porn) so we need to hide those sites. So "Would it be okay to look at it at work" is another factor.
The "Would it offend your grandparents" type test also happens.
There is also the aspect that while we do not actively stop people from creating these sites, we would rather not encourage either. Someone who writes and blogs for themselves is one thing (which would still be marked as mature but arguably the owners would not object because they are blogging) and others whose blogs exist to drive traffic elsewhere are completely different.
There cannot be a definitive line drawn if for no other reason than we all have differing ideas as to what constitutes Adult material.