freitasm: Back to what Talkiet raised. Why block pornography? Because someone doesn't like it? This is not good enough reason.
We already have a filter that applies to filthiest of filthy, child exploitation. We are safe. So we don't have to worry about man and woman, woman and woman, man and man, etc.
Seriously, it is quite hard to stumble upon this kind of graphical stuff in normal day to day browsing.
Hard to stumble upon by accident sure... but not hard to access on purpose.
The justification for it would be the same justification we have for age ratings and censorship of books/tv/movies currently.
However when you weigh that justification against: potential abusive use of a filtering system, parent's responsibility, available voluntary options... I don't think the pros/cons stack up for going ahead with it.