It's just because geek culture encourages cloistering and each community tends to lean heavily one way or the other. For ever nasty Return of Kings subreddit, there's a tumblr where feminism and LGBT inclusion are taken to the extreme, and everything in between.
It is extremely prevalent. No offense but Geek culture just attracts the worst of the worst. Gaf is one of the few exceptions, but overall people who are into geek culture are extremely misogynistic.
Yeah, I think what differences you do see on GAF have to do with the modding policies and attitudes rather than inherent qualities of the member population.
