So you are making an example where people with different attitudes to a competitive game get annoyed at each other?
Your example is actually quite good. It explains very well why those players in WoW that care about their performance use various tools and indicators to find like-minded players to play with and filter out those that don't have the same attitude.
I can assure you that the last thing a player in WoW who has competitive spirit, takes pride in doing his best and who has respect for the effort and time of his fellow gamer is to become involved in a pick up game (a pug) with players that don't share his idea of fun.
This is why we in Retail have useful tools like Raider.io, Warcraft logs etc.
And today in Classic people also use various tools to ensure that they play with people with a similar mindset. And people did the same in Vanilla.
And regarding your example:
What if the guy at the gym said: "Who wants to play? I only want to play with people that don't slack and give it their best." And then people joined who as it turns out didn't gave their best and obviously slacked. His anger in that case seems well founded and the "toxic people" in this case are the slackers.
On the other hand if that competitive guy joined a game where it was stated: "we just goof around" and he then got angry then he would be irrational and wrong.
So that leads me to my question once again:
What stops those players that just want "to goof around" to find other players that just want "to goof around" and exclude those players that have different ideas and values from their group?
From all the posts in this thread it seems that it is only "the goof around"-players in WoW that have issues with grouping up with like-minded people. And I am curious why it is so.