This is just hilarious:
While Google allows you to filter pornography and explicit sexual content
from your search results using their “SafeSearch” feature, they do not
provide a means to search exclusively for such adult material.
…
The unsafe search works by performing two Google queries, a normal query and
a query with SafeSearch enabled. The set of “safe” results is then
subtracted from the set of normal results to yield only “unsafe” entries.
