You aren’t the first to notice this. On average atheists may tend to be liberal and accepting of social change, but there are no guiding principles except not-religion, and that isn’t much to go on. On one extreme are those whose behavior very much resemble religious fundamentalism; the methods of argument they make against religion is similar to the arguments fundamentalists make about science and evolution.
There is worse. Back when I was running the big G+ atheism community we had some that could not tolerate our female moderators. Misogyny on parade. They were also terribly put out that one of our mods could have an open and friendly discussion with a Deist (also female). These folks split off to form their own group, but their antagonism never ceased.