"Only if you want to watch smut."
"Most Internet blockers block far more than smut because they suck at their job."
What about sites like reddit or tumbler? Do you just block the whole site? Block based on keywords and sub-strings? It's pretty hard to do. You'd need to do something complicated. Macs have a filter where you can white-list stuff, but some sites use services that do lots of load-balancing, and it's almost impossible to white list all those servers via the "load page, whitelist, repeat" method, and who knows what else those services serve?
For children, youtube has a mechanism to filter objectionable content, but it only works so well because there are some things that don't get flagged, but I don't want my young children viewing them.