It's my experience that atheists are constantly proclaiming how open-minded and free-thinking they are, but when you present them with even the slightest suggestion to the contrary, they quickly become ultra-defensive and close-minded. Most of the atheists I encounter are far more certain in their opinion that no gods exist than most theists I meet are certain that their version of God, does. Even though most theists like to proclaim their "belief in God" while most atheists will employ all manner of subterfuge to avoid actually admitting out loud that they don't believe any gods exist.
Well, they certainly do like to proclaim their own righteousness. But honestly, what group of humans doesn't?
Something no one ever seems to consider around here, though, is ... In the same way that it's not the blowhard standing in the middle of the bar telling everyone how tough he is that you have to be careful of. It's the quiet guy sitting down at the end that isn't saying anything that you want to steer very clear of. Because he doesn't need to tell anyone how tough he is. He just is tough, and he knows it.
Same is true in many cases among the religious. The louder they proclaim the certainty of their religious belief, the less faith they actually have what they are saying. It's WHY they are always proclaiming it.
There are a thousand theists in the world for every one atheist. So of course there is less sectarianism among atheists.
Also, there are endless ways of imagining the nature and existence of God/gods. There is only one way of imagining the non-existence of any gods. So what is there for atheists to argue about?