When it finds the most relevant sites to your query, Google also tries to filter adult sites, especially for general queries. There's a good reason for that: unless your query is explicit, you may not want to see adult sites in search results. Google also filters explicit words from Google Suggest or from related searches.
But not if you use Google China. Here are the related searches for girl at google.com:
If you search for girl at Google China and scroll down to the bottom you'll see different related searches:
Crack at google.com:
Suggestions for the query crack (Google China):
And here's something more language-independent:
So the question would be: why is it acceptable to show these queries in China and not at google.com?