Simple list comprehension
Well for starters with 

[self.addNSFWPoster(post) for sub in self.content for post in sub]


you are constructing a list of 

[None, None, ... , None]


with a length equal to

len(self.content) * len(sub)


that you are immediately throwing away because you aren't assigning it to a variable. In other words you should only use a list comprehension for building a list, not for calling a method that is doing work elsewhere. 

An alternative implementation is as you might expect a nested for loop:

for sub in self.content:
    for post in sub:
        self.addNSFWPoster(post)


To speed this up further you could reduce your function call overhead with the following:

for sub in self.content:
    for post in sub:
        print 'Parsing author and adding to posters.'
        self.nsfw_posters.add(str(post.author))