"I thought she posted it in an athiest sub-community of reddit?"
As I have said many times now on this thread, top posts on the atheist sub-community of reddit are also cross-posted to the main page of reddit. As I said before it is as if you posted on Feminism but your thread was also linked to from AIBU and Chat and then you use the content of that thread to slate feminists. Except Reddit is global and Mumsnet is tiny in comparison. The homepage of Reddit is visited by 0.8% of all global users of the internet. They serve 1.6 billion pages per month. 21.5 million people visit the site every month. That's an awful lot of potential contributors to that thread. In comparison the atheism subreddit has about 350,000 subscribers, so to suggest that the actions on that thread are the fault of the reddit atheists when millions of other users could have seen and contributed to it is a bit of a stretch.
"And do 15yo girls really post photos of their faces holding books in order to illicit men to say they want to kidnap and rape them, in very graphic language?"
No, of course not, and I have never said that. I have said that rape threats would and should not be the expected response. I have also said that a jolly thread soley about atheism would not be the expected response. However, women on reddit do on occasion post photos of their faces holding geeky books in order to elicit responses which sexually objectify them and in order to get upvotes based on their attractiveness rather than the purported topic of discussion.
"It's exactly the same as saying, well she went out in a short skirt after dark"
No it's not. It is saying that she went to a particular site on the internet with particular rules and a particular way of doing things and posted in a particular way on that site which would be recognised and responded to in a particular way, so wondering why she got a non-atheist discussion response to a reference to her anus is silly when the answer is obvious. She also got plenty of atheist discussion, btw. And she also got horrific rape threats which is completely unacceptable and which have been condemned by many on reddit and which have also been condemned by me, a point which some on here seem intent on missing.