I think this is painting a rather stereotyped view of people on the left. I share most of the views of the left, however don't end relationships with people over it, preach, or attend any seminars. I think you're seeing the vocal minority here, which is of course more outspoken as they care enough to talk about it. Apart from the most extreme people, I have had many productive conversations with people whose views were more left than mine and haven't been ostracised once.
You are of course right that most people aren't like this. This thread is about those few who are, and how they can end up dividing everyone else, unwillingly, into accolytes versus enemies. It's not safe to say "I am left/center/right/whatever but I don't think this is the way to go about it" around this type. It is a separate axis from left-right, and is maybe correlated with the authoritarian-libertarian axis.
