The problem with this is if you blocked out every website that had the word "sex" in it, it also blocks out every site with family name Middlesex or the word sextant in it. It also blocks out every subscription form that asks you for your sex(2).
Software that relies on lists are hard to update because of the 4,200 websites added each day. Many software programs post updates which parents can download, but no one, no matter how hard they try, can keep up with the daily addtions.
Additionally, these sites are rated and added to the list by people who may not share your values as a parent.