It may that they did not add it for a reason also. There are a lot of edge cases because many potentially dirty concepts are made up of words that are not bad alone. For example a text can have both "girls" and "nude" in it without being vulgar, but if it has the phrase "nude girls" the chance for it being pornografic is much higher.

( Searchdaimon have done some research on this and have a list if anyone is intrested: https://github.com/searchdaimon/adult-words )

There is also the data analysis perspective: http://qr.ae/8W4Pz1 :)