What I'm suggesting is that if the moderators do nothing about it, then the community will decide on its own what is on-topic.

What is de facto on-topic does not need to be related to the subreddit name. For example, on /r/blackatheism any minority is on-topic (even though it was initially meant only for blacks)

Right now, the community is deciding that images such as this one is on topic. I am pleased that you are expressing your disagreement, though.

That doesn't mean that anything LGBT related is automatically relevant to atheism.

I think that it is increasingly obvious that at least some LGBT topics are considered relevant to /r/atheism - at least according to our community.

Right now /r/atheism is not about just atheism. I say that not only due to the broad language of the sidebar, but also because of to the way our community accepts such material.

I agree that not every LGBT topic will have as much acceptance, and someday this might change entirely; but right now, I don't see the position you present - of rejection of posts such as this one - as some sort of community policy.

edit: I see that most commentators actually agree with you. But what makes the topic go up isn't dissenting comments but actual votes. One might dislike this democratic aspect of reddit - and maybe it is appropriate for the moderators to remove such material. It's up to them though, because if they don't act the de facto policy of this subreddit will probably continue to be the same.

This is usually a problem for default subreddits.