So the Wikipedia write-up is here, but basically:
Rebecca Watson, a prominent figure in online atheism who focused on feminist issues, attended a conference in which she gave a talk about how women in the atheist community often felt objectified and faced sexual harassment.
After the talk, she hung out with some other attendees in the hotel bar until about 4 am. When she left to go back to her room, a man followed her into the elevator and asked if she wanted to go back to his room, which she declined.
She later published a video where she shared this experience and said "I was a single woman, in a foreign country, at 4 a.m., in a hotel elevator with you—just you—and don't invite me back to your hotel room, right after I have finished talking about how it creeps me out and makes me uncomfortable when men sexualize me in that manner."
This triggered a severe harassment campaign against her and caused a deep divide in the online atheist community. Several prominent figures - most notably Richard Dawkins - mocked her for acting like this was a real incident of harassment or started whining about how "oh so men can't even ask women out anymore or it's basically rape?"
This is, I think, where a lot of the "atheism has a huge sexism problem" trope comes from, but it's worth noting that almost every blogger I knew in the movement supported her and Dawkins faced a huge backlash for his comments, which led him to half-assedly apologize.
And the thing is, like the last anon said - atheism *does* have a sexism problem, because society has a sexism problem. There is not a religious or ideological group that doesn't. Acting like it's uniquely atheist to harass or disrespect women is absurd. But people like to point to Elevatorgate as "proof" that atheists hate women, while conveniently ignoring the ways in which religion treats women.
