People become attached to groups or subcultures in an effort to feel like they belong. Psychologists call it "affinity-seeking behavior" that affords a sense of validation. Most people are social animals, seeking to belong somewhere, in some group, among "friends."
Many subcultures, cults, or groups must adhere to specific tenets. Successfully passing regular, ritualistic loyalty tests provides validation which reinforces the psychological bond to the organization.
When a member is confronted by someone who has opposing beliefs, they fear that the influence may disrupt their performance or their standing within the community. Their biggest fear is banishment or rejection. They must reject arguments against their own belief systems, even at the cost of logic or reason. The only way to reach someone under such influence is to find some little things in common, and keep them focused on those things. Gradually steer them toward critical thinking skills and logic.
Feminism as a topic for discussion at large atheist gatherings makes perfect sense because atheism is supposed to reject all forms of gender role distinctions, because they are codified in religious doctrine. It's perfectly reasonable to raise awareness of misogynist behavior and propaganda because it is founded in the psychological influence of religion over many generations. Despite self-proclaimed atheism, some people are still shackled to certain assumptions that were formed from exposure to religion.